For the past few weeks the children have been insisting on having food dye in their baths. I am finding that is really helping with them understanding colour mixing/blending.
I get two clean juice or empty milk bottles and fill one with one primary colour (for example yellow), and the other with another primary colour (for example blue) and I ask each child to wait their turn to empty their bottle in the bath (ta-da, green water). The children find it pretty spectacular and somewhat magical. It doesn't seem to stain their skin or the bath, so everyone is happy and they are learning while they play.