You can find instructions on io9 to make this neat drinkable rainbow. You alter the density of water using either Skittles or sugar and food coloring. Neat! They describe using just a few tablespoons of water which would probably work out better in a large test tube – I assume this can be scaled up using more water and more Skittles/sugar to make the beaker-sized version in the picture below.

What you need:

Five small cups for mixing
A clear glass
A wide spoon
Skittles
2 red
4 orange
6 yellow
8 green
10 purple

To do:
Fill five cups with 2 Tbsp of water each.
Dissolve the Skittles, each color in a separate cup. If the candy is not dissolving, stir frequently or heat the water. (The waxy film floating on the surface can be removed or ignored; it won’t affect the experiment.)
Pour the purple water into the clear glass.
Hold the spoon upside-down over the purple water, with the tip of the spoon touching the edge of the bowl above the waterline. Slowly pour the green water down the back of the spoon, so that the green water does not mix in with the purple. Instead, it should float on top.

Alternative method: use a small syringe to make the layers. Suction up the green water, hold the tip of the syringe against the edge of the glass, and gently squeeze the water out to make the layer.)
Repeat with the other colors, and admire your rainbow.