This was a simple color mixing activity I did with my kids using watercolors.
black pen (waterproof is even better)
Draw a simple triangle on watercolor paper.
Wet the paper well with a sponge or paintbrush - try to avoid the triangle line or use a waterproof pen.
Using a paintbrush put a primary color at each corner of the triangle: red, blue, & yellow.
With a well saturated paintbrush, drag the colors between each point so they mix to make the secondary colors: purple, green, & orange.