How does shader work?

[pa3ck]

Hiya. I was looking at shaders, but it just won't stick into my head.What I want to do is, I have a dff and txd for a vehicle, but I want to apply shaders for different vehicle elements. I mean, I'll have the same texture but the number is going to change on it ( on the texture, probably I'll need an image file? ). Is it possible to do it without having a seperate texture for every number? I don't know, if I clearly explained it to yous, but that's the best way I could explain.

Just like this topic: viewtopic.php?f=91&t=74142&p=685257 . I want the same thing, except, I want the shader to be 'flexible' so that I can set my text or number in-game.