When you begin, you should have just one layer called "background" in italics. (It's in italics because it is a "flat" layer.")

A flat layer is like a piece of paper taped to your desk; you can draw on it, you can paint on it, but you can't put another picture underneath it. Since we need for the colors to be under the lines, we'll need to make it into a "free" layer. (Which is basically like removing the tape.)

To make it into a free layer, you need to duplicate it. So select the background layer, and drag it onto the "create new layer" icon. (The one below it that looks like a piece of paper.) This will make a duplicate layer called "background copy". (Note that it isn't in italics.) You can now throw away the original italicized version, by dragging it onto the little trash can.

However, even though this layer is no longer "taped down", it's still an opaque piece of paper. Now we're going to make it translucent, like an animation cel. To do this, all you have to do is set the mode to "Multiply". (The mode selector is right above the layer, and starts out saying "Normal".)