with both adobe and gimp you do what neebick said. Create a layer on top of the layer you want to color. Choose a color. Set transparency of that layer. But this will basically combine the colors under the layer, not really turn it a certain color.
In photoshop (probably in gimp to) the the better way is to apply a style or something like that. There's a way to tiny it. I'd have to look it up again. Ahhh, it;s blending options/color overlay.