For Kindergarten I try to use transparent paint. When the Kinders paint, they often don't stay within the boundaries of their drawings, this paint allows thier drawings to show through!
One of my favorite paints to use is Sargent brand Washable Glitter Paint. The colors are brilliant and the paint is transparent. If glitter isn't your thing, or you have a tight budget you can mix up your own transparent paint:
1 part Sax True Flow Tempera Extender
3 parts water (or sometimes more water depending on how thick or thin you want the paint)
Liquid Watercolor to reach the deisred shade
Crayola Glitter It of Pearl It if desired
Mix together and go crazy!