Remove the label from the bag using the nail polish remover on a cotton wool ball.
Give it a wipe and allow the nail polish remover to evaporate.
Place the bag on your surface and fill with some red, yellow and blue paint.
Seal the bag and then tape the seal closed.
Secure the bag to your surface (I used a white sheet of cardboard from a cereal box so that if any paint did come out of the bag then it would be off my table).