To add detail and realism to your models, SketchUp enables you to paint materials on faces. Materials are essentially paints that have a color and optional texture (defined within an image file).
If you know the HSB, RGB, HEX or CMYK numerical code for the color you want, simply type the color values in their respective boxes. Or, if you find a color you want to use elsewhere, within or outside SketchUp, take note of its numerical code. Then you can simply retype the code for an exact color match.