Place some coloured crystals under microscopes. Illuminate them through a Polaroid filter to show further colours. Look out for compounds of iron (red), copper (blue) or nickel (green), and yellow sulphur (does this give a clue to the name Yellowstone?).
Simple chromatography of green plant pigment might help show the browns and yellows of carotenoids. Crush leaves or grass and mix with some ethanol (caution: it's flammable), which is best warmed by in a test-tube placed in a beaker of hot water. Filter out the green solution and stand a piece of chalk in a few millilitres of the solution in a container and cover in Cling Film. See the solution rise and the colours separate into bands of green, brown, orange and yellow.