Explain to your preschoolers what makes rainbows. Describe how when light reflects off of mist it brings the colors out of the light. If the weather is accommodating, you can go outside and create a rainbow with a garden hose. Make sure that the sun is at your back and that you can get a fine misting of water with the hose. You may need to wave it back and forth to catch the light correctly to make a rainbow.
Give each child a paper plate and coffee filter. Place red, yellow and blue food coloring in the middle of the table. Help your students place a drop of yellow on the coffee filter and then add a drop of either blue or red on top of it. Talk about what color appeared. Encourage your students to experiment with all three colors to see how many colors they can make. When a student is finished, ask her to compare her work to a picture of a rainbow. See if she notices that orange falls between red and yellow and green between blue and yellow. If not, prompt her to find the colors she has made and to theorize what relationship they have to a rainbow.
Preschoolers frequently enjoy playing with rainbows. Bring in a box of prisms and encourage your students to hold them next to the light to create rainbows. The light you use can come from a window, lamps or a flashlight. Consider having them work in pairs. One student can hold the prism so that it casts a rainbow over a piece of white paper and the other can color it in. Or hang the prisms in such a way that students can catch the rainbows on paper and color them independently. Discuss why prisms don't work in the dark with your students.
Some of your more curious preschoolers may ask why rainbows are shaped like an arc. The answer is because water acts like a prism by bending light as it travels through it and the different colors in the light bend differently, so they appear curved. This also explains why the colors always appear in the same order. Since this is a very abstract idea for preschoolers, show your students that water creates the illusion of bending by putting a pencil in a glass of water. It will appear bent because the water bends the light and creates the illusion.