First read the book "Little Yellow, Little Blue" by Leo Lionni. Then discuss how two primary colors, yellow and blue when put together make green (which is a secondary color). Discuss other primary colors that make secondary colors.
Then take them over to the art center and pour yellow finger paint into one aluminum pan and blue finger paint into another aluminum pan. Have the students press one hand into the blue paint and then press their hand onto the paper. Then have them press the other hand into the yellow paint and press it down on the paper next to the blue hand print. They will then take their blue and yellow hand and rub them together until they turn green. Once they are green have them press them down on the paper below the yellow and blue hand prints.
Once they are dry you can write next to the hands "One blue hand and one yellow hand makes two green hands!" You can use other colors to make secondary colors. Again, it is a great project to reinforce the concept of secondary colors.