I'm sure there is a gl line for this but can't find it anywhere, basically I have a texture loaded (loads of characters) and I am looking for a way to change that white/grey colour in the texture to a specified RGB on the fly...

So basically:
Render a texture that is basically black and white and use the 'greyness' to indicate the shade of the RGB colour to draw whilst keeping the alpha value. If it can be done at all...

Any help would be greatly appreciated, because I have no clue on what gl calls do the job efficiently.