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 [wiki:Image_Converter 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 [wiki:Colour_Tester Colour Tester] tool lets you examine textures in any colour, to ensure they are working as expected.