Oh look, it's a fossilized cheap trick. "Clipping Mask."
It's a tracing! Plagiarise yourself!
Text-only half-assed tutorial type thing:

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Photoshop. (I have CS3.)
2 blank layers. (1st and 2nd layers.)
Black background underneath all. (3rd layer.)

Draw rainbow on 1st layer. Hide this layer.
Draw skull on 2nd layer, in white.
Do nothing on the 3rd/background layer.

Unhide 1st layer.
Right click 1st layer in "Layers" panel.
Click "Create Clipping Mask".
- The 1st layer will join itself to the layer directly underneath it. (2nd layer, in this case.)

Ta-da. Rainbow skull. Beginner tutorial.

If you don't understand, figure it out. Work your brain/Google muscles. Make some happy mistakes. Let me know what confused you. Ask me questions if you want to.

There are other ways to use the Clipping Mask, and other things you can draw. Try stuff! Explore! Break my rules!