This one will take some physical effort as it's better to leave the house to do. It's so fun though and great for your little one to explore and learn their colors!
You just need a white paper, colored markers (or crayons) and some scotch tape.
Color in patches of different colors. Try to do a good variety and space them out. (See pic for the example)
Then just walk around your backyard or neighborhood and find some fun things that match the colors. Flowers and leaves work great, rocks too! When you find a match, tape it next to the designated color. This way you can bring home your hard work!