That depends on how you have built the shaders. Just some of the options would include:
If you are using color or color gradient in the color channel then you can adjust the color or color keys.
If you are using texture maps you can adjust them in PhotoShop, the GIMP, etc. If not, and you want to use this method, then you can export the shader or bake it. If you export the model Carrara will give you the option of including texture maps for the shader (IIRC it is a texture map per shader channel).
If you have Fenric’s wonderful Color Balance then you can use then to change R, G, or B values and/or hue, saturation and level.