Changing Material colors on a mesh

How can I set up a material network that will allow me to change the color of decals or painted strips on my vehicles while retaining the color of the paint below the decal? For instance, I would have a blue car and a decal that can be changed to colors chosen by the user. The car would remain blue but the decals would be user chosen.


You can use masks for mixing textures or change the color of the desired area.