12. Now we are ready to paint the marble. Target the composite channel--the one at the top of the Channels Palette--by clicking on it. At this point you may want to check the Layers Palette to make sure you are painting in the layer you intend. Fill the image with a light color. This will be the base color for your marble.
13. Now we will want to lighten the marble a little. Load the first of your Channel Masks as a selection.
14. Go to Select > Inverse. This is important. If you don't inverse the selection, you will lighten the wrong part of your image.
15. Fill the selection with a percentage of white. (Here I used 100%).
16. Load the next Cannel Mask in the series. Click on the Foreground Color Swatch in the Tool Box and, from the color picker, choose a color that is slightly darker than your base color. (I usually select a color a little below and to the right of the previous color.) Fill the selection with 40% of the new color.