I absolutely love mixing secondary colors with kindergarten - it's magical.
First I read them the book, "The Dot" by Peter H. Reynolds. Next, I pass out long thin sheets of paper with black crayons and ask the students to write their name and draw some dots - just like the book. After students have some dots I pass out watercolor trays with only yellow, red, and blue showing. The students jump right in and start to paint colored dots.
It doesn't take long before someone gasps and says, "Ms. Novak I made GREEN!" All the other students get excited and I ask the student how they painted green when they didn't have green on their tray. As I talk with this student I can hear other students discovering how to make green, purple, orange and all ranges of these colors.
The excitement and awe in their voices as they discover secondary colors is absolutely amazing.