You can use plants from your yard/neighborhood to make your own water colors paint.
Experiment with different plants to see what colors you can create.
The process is simple.
Collect various flower petals.
Place the petals and warm water into a Ziploc bag.
Use a rolling pin (or just squash around with your hand)-- you should see the plant coloring the water.
Leave the petals in the water for a while to allow more color to steep from the petals.
Use your new paint to make a picture. Let me know what kinds of plants you used! Take a pictures and submit.
Here is a great webpage to help you make your watercolor: http://www.learnplayimagine.com/2013/08/homemade-paint-natural-watercolors.html