Step 1: Spread shirt out on waxed paper or newspaper-covered work surface. (You may want to work outside or in the garage.) Place waxed paper between front and back of the shirt and inside the sleeves.

Step 2: Cut fern fronds and arrange on front of the shirt. If desired, lightly touch a glue stick to any leaves that are sticking up. This isn't necessary, but it helps make a clearer image when painted.

Step 3: Shake brown paint well. Spray in a diagonal area at bottom right of shirt. Repeat with green paint, slightly overlapping paint colors and continuing up in a diagonal direction.
Continue pattern with blue paint. If necessary, use brown again to fill in extra space at the top left corner of the shirt.

This technique could be used on other products.
Source: http://home.howstuffworks.com/decorate-kids-t-shirts7.htm