Now go the the Channels Tab.

Create a new channel, and name it Flats.

TIP: Remember earlier I said that I start my process of coloring in Grayscale mode, that's why you can only see the Gray channel

Paste in the merged layers we copied in this Flats Channel, and you're finished.

You have now created a Flats Channel that you can use as a guide to your coloring.

Now the only thing to do is erase the Basics layer in the Layer Tab, convert your file to RGB and start coloring.

If you want to see how I use the Flat channel, view my Hair Coloring Tutorial