Color mixing in any variety is always a hit at our house. Our friend Emily taught us the trick of using ice cube trays. If you don't have those, plastic egg cartons, paint trays or even muffin tins would also work. We used liquid watercolors to make a few different starter colors for S. You could always use this as a way to teach primary colors and just include red, yellow, and blue. S has done this so many times, she prefers a more exotic set of starting colors.
I left several cubes as blanks (just water) to give S more freedom to mix her colors.
Here she's pointing to a shade of pink she made that is "her favorite ever!"
Using eyedroppers is a great way to get in some fine motor practice. It also makes the activity last longer. Every time she does this, S manages to develop new shades of a variety of colors—it really never gets old in our house!