You need three sheets of paper for this -- one in any color and one each of green and brown. Tear the brown and green into small squares (kids love to help tear) and with the remaining sheet of paper draw a simple tree using a brown and green marker.
Time for some color matching and tree creating. My two-year-old used the glue stick and put the brown squares where he saw brown and the green where he saw green. (I know, very simple, but it was perfect to give my little guy much confidence all while practicing his colors.)
You could make this more challenging by adding new drawings and more colors. We started out easy and fun.