This is a pretty easy and fun look for whatever you are willing to attend with rainbows on your face.
To do this look, you will need:

-Eyelid primer
-Eyeshadow in violet, blue, green, yellow, orange, and red
-Glitter or black eyeliner
-Mascara (optional)
-Bright lipstick (optional)

Step 1: Primer

Apply makeup primer to your eyelids from line to brow. I used Urban Decay's Eyeshadow Primer Potion.

Step 2: First Color

Wet a small makeup brush with water, then cover with the violet eyeshadow. Apply a small streak to the inner corner of the eyelid.

Step 3: Top Lid

Using the same wet-brush technique, carefully apply the rest of the colors evenly across your eyelid in reverse "rainbowrical" order.

Step 4: Under Eye (optional)

You can create a more elaborate look by repeating the same look under your eyes, except in the opposite order as the top lid.

Step 5: Eyeliner

Complete the look by adding a cat eye to your lids with a glittered or black liquid eyeliner. You can also add mascara, or complete the look with a bright colored lipstick.