Honestly, I don't too often ask kids to do too much with color theory but a friend asked and my three year old and I do actually do some, though I hadn't ever considered articulating it.... So here goes.
Lots of books and seeing colors
My favorite color theory books for little little kids are
Blue Goose
Blue Goose is for younger kids but is sooo well done.
There is a 3rd book I'll come back and add if I can find a link. I think it's out of print but it has overlays and mixes colors
The Little Engine That Could
This version specifically mixes the yellow sunset with the blue train. My three year old asked why the train is green so we spent a good amount of time with this one and memorizing basic color mixing.
Color Schemes
If you want kids to play with a color you can pick a color scheme for them to play with.
Primary colors - red, blue and yellow. Giving kids two to play with is a fantastic place to start
Analogous - colors next to each other on the color wheel.
Warm or cool colors - think the sun (yellow, red, orange) or ocean (purple, blue, green)
*Note*
The color schemes can get tricky. Red paints often have yellow pigment in them so purple is hard to mix. Purples can actually be warm. But for the younger kids this isn't super important. I typically teach the standard and don't bring up the debated primarys or other such things unless I'm with older kids or doing a one-on-one
Artist Study
Study what colors artists use.
Vincent Van Gogh's Sunflowers are full of warm colors and his Stary Night is mostly cool colors.
You can cut tissue paper to match a color scheme or only include certain colors.
But Most of The Time
I do let kids pick their colors and we get a lot of browns. As kids get older or if they want to improve their skills, I will often tell them to stop mixing early on.
