Unless you give them two colors at a time, young kids tend to use colors as they come, rather than mixing them. Too much direct translating from markers and crayons?
Asking for 100 colors is really a stretch for kindergardeners, I think just thinking about the possibility of 100 different colors is expanding for some of them, so this was a really great activity. We got the idea from TeachKidsArt: Mixing 100 Colors. It also goes with all the 100 activities that kinders and pre-schoolers are doing in the spring, the 100th day of school, practicing counting to 100, push it into art too.
I really had a lot of fun with this too! As it doesn’t take ‘skill’, just patience, it is something that parents and children can work on in parallel, on a relatively equal and companionable footing. (‘Just’ patience. Ha. Well, that is one of the major skills that this activity will stretch.)
I did find though, that the washable tempera paints are MUCH less suited to color mixing than student (non-washable) tempera paints, we tried both. The student tempera has about 10x as much pigment, so your mixed colors come out much more saturated. It is quite a challenge to mix a real orange using red and yellow OR magenta and yellow with washable tempera, whereas the same exercise with student tempera gives you a nice orange.
Here is a quick little PDF with light dashed lines outlining 100 boxes to fill with colors. Teaching Kids Art used very light filled gray squares with white separators, but I thought our kindergardeners and preschoolers needed a little more contrast, especially since we do our art outside in the sun rather than in a classroom.