My son loves this game / activity and can entertain himself literally for hours. All you need is food coloring. I got this 4 color, food coloring set at Target for $2. I gave him a dropper from one of the first aid kits we’ve got at home. I put water in few old baby food jars, then put a drop of color in each. I also gave him few empty ones. He picks the colored water up, mixes them.
This activity is not only about mixing colors but being able to pick up the water with the dropper – motor skills – as well. I really don’t hover to tell him what colors make other colors when mixed etc. Our table is glass, so I don’t really care the colored water spilling on the table. I just let him play by himself.