I used poster paint to color the flag. I actually had my kids help me with this, since I wanted it to look rustic and therefore a perfect paint job wasn't necessary:
I diluted brown paint I used on my kitchen table and glazed the entire board to give it an aged look. I also splattered it with a little oil rubbed bronze spray paint.
Lastly I sprayed it with clear coat so the paint wouldn't run when it gets wet (since it is going to be outside all summer)
Not bad for an afternoon of work. Total cost: ZERO!