With standard Diplomacy 'How to play...' articles having been rewritten over and over again, I want to put forward my own opinions on how to play each power in Colonial Diplomacy. I have never finished a game of CD, so I am only using my perceptions based on games that are ongoing.
The British may start with more units than any other power, but it is inherently weakened by the fact that three of its units are at least three moves away from the central hub of the British Empire - India, as well as at least 4 moves away from each other! This means that for the first two turns these three units are acting totally independently of each other, and they are in the sphere of influence of four separate powers. Sudan is the only 100% guaranteed build, but this lets Turkey into one of the most precious supply centres on the board - Egypt.
So what do you do? Who are your real enemies and who are your best allies? How should you treat each other power?
China: Despite both the British and China having many options they will often come to blows over the Kashmir/Punjab/Tibet/Bengal line, the main reason is that one side or the other is so heavily involved elsewhere that the temptation to grab the others centres is just too much. China should be more of an ally than an enemy, both powers have too much to lose by fighting each other, and who is going to gain much from the war? Both sides need a host of armies to push into the others home centres, meanwhile the other players take advantage by sniping at your less well defended areas. The fleet in Hong Kong is to be 'given' to China who starts the game without a navy, it can be used to stem the ever-present menace of Japan or used to eliminate the French very quickly. Eliminating anyone very quickly in Colonial Diplomacy is nowhere near as easy as in standard Diplomacy, there are just too many bolt holes - one of the attractions of CD.
Agree to keep to the line drawn on the map, Bengal is British, the eastern gateway into British India; Assam and Upper Burma should be Chinese, but only if you are to get anything out of the French Empire. Don't worry too much about China taking most of Indo-China as, by this stage, the Japanese and Russians will need to look at Chinese centres for expansion, irrespective of whether they were allied to China or not.
Co-operative moves against Turkey and Russia should be reasonably successful in the west, but in this case you want to be taking the majority of the centres. The Himalayan no-go zone is similar to Switzerland in the standard game, it acts as a fulcrum for the movement around it acting as the centre of the board. It is possible for the British to win without stabbing his Chinese ally, so the alliance could be very powerful - Britain provides the fleets, China the armies, a classic combination for the two way alliance.
Dutch: The Dutch are probably the British players worst enemy, it won't take long for him to take Singapore, and he can take Ceylon in the very first build turn. In fact the Dutch are the only power to absolutely guarantee 2 builds in the first build turn, though China should be able to do it too. But what if you feel an alliance could work - fool!!! The trust required is one that will test the most steel-willed of players, the Dutch must go north against Japan and China while you concentrate on the west against Turkey, the only problem is that the large concentration of supply centres in the south-east quarter of the board will be easier for the Dutch to reach, and when he starts getting bogged down against Japan he must stab you. It is possible for the Dutch to win with fleets alone without coming west of Sumatra, but in practice this would be almost impossible. No, the Dutch must be one of the early targets. Give up HK to the French in exchange for help against the Dutch, he should willingly oblige as his home centres are required for a Dutch victory. Make peace with Turkey, which will in turn divert Russia, and go for those black islands in the corner. Agree to keep Java Sea free allowing, with wry consent, that he can take Ceylon, then move into JS on turn 1. You now have all three Dutch home centres at your mercy and he won't cover all of them otherwise he risks no builds! Ignore Malaya for now, your sole intention is to get a Dutch home centre. If you can really trust Turkey you could move Aden to GOA and into SIO in 1872, or move Bom to ARA.S and then to SIO ignoring Kar, no other unit is likely to go for Kar but you will have shown Turkey that you really want that alliance. The French should open to GOS allowing you to get into SCS, from where you will go for SUL.S. This may seem to be an odd way of playing the British, but if you don't you will be the meat in Dutch alliance sandwich. If you don't get JS straight away you will have stood a Dutch unit out reducing his possibilities for builds, then go for Malaya and build another F(Sin)! F(Mad) is another must for a war against the Dutch. Once the Dutch are out you can be comfortable in the knowledge that the east is virtually sewn up and you can look forward to enemy free seas in the Indian Ocean.
French: For 'French', read 'Friend'!! He is the weakest country on the board from the start of play, no guaranteed builds and surrounded by three potentially hostile powers. The French should be easy to trust as he will want that pesky fleet of yours out of his hair, offer him the use of the fleet or even Hong Kong itself if he is worried about you building there. An alliance against the Dutch should be the priority as long as no French fleets wander into the eastern side of the Malay peninsular in the ensuing conflict. Co-operation against China can also be considered, particularly if he is under pressure from his northern neighbours, most of the Chinese centres are in the south so you should pick up the main share of the spoils. I think China can collapse very easily when pressured from all sides. The British and French are the poor relations in the eastern seas, so sticking together can prevent the rapid expansion of Japan southwards, or the Dutch northwards. In any situation there should be agreement over South China Sea, otherwise both parties will get nowhere fast, perhaps it is best to allow the French out first as the following turn is the build turn and there can be no backstabbing by the British in a French home centre.
Japan: In the far north east corner sits one of the strongest powers on the board, thank goodness he is the furthest away from you! There is little interaction between the British and Japan in the early years, though the fleet in Hong Kong can upset him by standing him out of Formosa or acting as an aid to China. On balance Japan should be 'a good thing' for the British as they can annoy Russia, China, the French and the Dutch - possibly all of them in one go - and get away with it! Encourage him in whatever he does, the centres that Japan wants are not necessarily the ones that the British want, there could be conflict in the Indo-China/Dutch home area but the likelihood is that other powers will have played a lot bigger part in the outcome of the game before the two forces meet in numbers.
Russia: Potentially the best alliance of all! Britain controls the south while Russia controls the north, what could be simpler. A pincer strike on Turkey will eliminate him and end the conflict in the western 1/3rd of the board in a very short time. So long as Russian fleets are not allowed through Suez you can leave a minimal garrison and both trundle east; Russia into China or with China against Japan; the British into the Dutch East Indies and South-east Asia. Both powers could reach 24 centres quite quickly in unison, but the reality is that the Japanese islands, Philippines and easternmost centres of China might be more difficult to obtain. This alliance can easily last all game with one side or the other snatching victory in the nick of time. One of the problems is that the Middle Eastern centres are thin on the ground and disagreement over them could cause friction. Bag, Shi and Kar should be British, but if you have to concede make Kar and Egy the very last bastions of your western fringes.
But what if Russia is an enemy? This is likely to be because you are supporting Turkey or China. The Afghan corridor becomes the front line that is difficult to cross by either side, if you think Russia is going to be unfriendly why not go from Del to Pun to Afg?!! This gets across the potential stalemate line straight away and gives you and your ally (Turkey or China) a head start on the attack into Russia.
Turkey: If there is one alliance that looks impossible it is the British-Turkish one. But, it can be most rewarding from both sides. Every game I have seen has involved conflict between the British and Turkey just because the British must hold Egypt and the Turks must take it in order to expand eastwards. Why not ally against Russia? The main reason is that Turkey picks up Rum, Ode and Mos and thats it! But is it? Look at the map. It takes three moves to get from Per/Shi/flag to Bom, it also takes three turns to get to Kag from Mos, only two from Per; so why the urge to go via the sea rather than by land. Due to the majority of the centres being concentrated into the south-east corner of the board, it seems logical to try and get to them via the shortest route, the problem is that by the time you do break through into the vastness of the Indian Ocean, the British have been reduced to a squirming puppet of one of the other powers and the Dutch are convoying armies into India like there4s no tomorrow!! You then have to look elsewhere anyway, so why not go north in the first place? The British should point this out to Turkey and agree to divide the spoils of the Middle East between them, ensuring that a British army garrisons Egypt and the Turkish fleet that started in Bag is either eliminated or decommissioned in Shiraz. It is vital to prevent any further fleet builds in Bag and encourage Turkey to move armies north with your support. If Russia is collapsing in the west he is probably doing likewise in the east and the way is fairly clear for a Turkish push into China while you look south-east and consolidate your gains against the Dutch!?
If Turkey is aggressive you have no option but to attack him, in this case get Russian help to crush him. You are on to a loser if the Turks and Russia ally as the ensuing stalemate bogs down useful units that are really needed elsewhere. Summary
As Britain you will want to try and be as friendly as possible to everyone while most of the other powers will want a piece of your estate. The Dutch will be your downfall if you ignore them, the Turks must be lured away from their standard anti-British stance and be encouraged towards Russia. China should be fighting someone else, so encourage Japan in that direction all the time maintaining a non-aggression pact with him. The French are feeble and open to any signs of friendliness, he could be your main man! Don't be put off missing obvious builds in the first couple of build seasons, go for position not pieces. You can even give up Hong Kong in return for favours elsewhere, but try not to lose Singapore, it is more important than it first appears.