To help differentiate units on a battlefield, most unit textures should make use of player colour: parts of the texture are marked as player-colour regions, and in the game they are colourised to match their player.

This is controlled by the alpha channel of the texture - high alpha regions are unaltered, low alpha regions become strongly coloured, and anything in between gets a proportional amount of colouring.

The Image Converter tool lets you edit BMP files where the alpha channel is represented by a black-and-white lower half of the bitmap, which is easier in some painting programs.

The Colour Tester tool lets you examine textures in any colour, to ensure they are working as expected.