My son and his buddy made Handprint Father's Day Cards last week.
Each boy painted their hand and placed it on the front of the card. I wanted something more than just a plain handprint so I gave them blue and yellow to paint their hand with (see last 2 photos below).
You could do this with any of the primary colors (red, yellow, blue) and the areas with both colors will mix into the secondary colors (green, orange, purple).
The card reads "My Hand" on the front and "-some Daddy(or Papa)" on the backside of the front cover. The boys traced each letter with markers and drew a picture of their fathers above the "-some Daddy" words.
Father's Day Cards made by our 3 year olds: