One of my favorite things to do with children is to point out the colors all around them in nature. A flower is rarely just one color. A tree trunk is not just brown. And my favorite are clouds – I love color in the clouds!
For this idea, go on a walk together outside. Pick a flower or leaf. (You could even do a rock, if you are studying them). Bring it into class and have the children paint or color a flower on their own, or trace a template for them to color. Make sure they add in all the colors to their drawing. Preschoolers often need encouragement to use more than one color. This will help them see why. You can use an activity like this for many different units of learning. Add science facts if you like.