I wanted my kids to go beyond basic color mixing and get to know some of the tricks to getting custom colors, but I didn't want to do the same boring primary-to-secondary sets. So I tried out a new experiment sheet and called it Discover Color. It is pretty open ended and the kids find mixtures they like (and don't like) based on some simple instructions - there is a place to record results and write notes, including a "Yes! or Yuck" vote. My classes are 45 minutes and this filled one session perfectly, but I can see using this before any painting project to work out a palette selection. They had fun with it and created some really nice colors!
If you'd like to try Discover Color, you can find it in my Teachers Pay Teachers shop!