If i have a textured template (like the ones with the creases and realistic effects), how do i select a certain part of the kit like just the top and change the colour inside that box so the white with the creases turns to say a blue crease or something?

you need a new layer, fill it completely blue (or just a part of it by selecting it) and multiply this layer. all of this you make in the layer window.

oh i just worked out it was ctrl+u