Here is another simple but very fun for the kids activity. They very seldom tire of it and we do this activity multiple times throughout the year.
I provide them an icecube tray (white or transparent works best) and fill three slots 3/4 way full with water and color them with primary colored food coloring. Then I set up paper towel or coffee filters and give them an eyedropper. Let them go. They'll mix colors within their icecube tray and I encourage them to drip some of the water on their towel or coffee filter. As you can see, the colors spread on decent quality paper towel, as well as coffee filters. With coffee filters we create little projects like coffee filter butterflies.
For the winter, we vary it just a bit and provide 3 cups of primary colored water and place them inside of a container of snow.
With snow, we also use washable markers and soap pump bottles with colored water. Ice cream scoops add another dimension to their play as they create different "flavors" of icecream and scoop it into bowls. I hear we are in for another snowstorm...might be an opportunity to try it out! :-)