How about a stencil? Buy one or make your own using the freezer paper method.(draw on the freezer paper and cut out your item -for example, an apple - cut out the apple leaving the outside intact.) The paints are acrylic, and about $1.00 per color, a couple of facial sponges. Iron the apple shape on your bag and fill it in using the red acrylic paint on the facial sponge. Then draw a stem and cut out the center - iron it on top of the apple and sponge it brown.
Cut out one or two leaves, iron them where they belong on the apple and sponge on green paint. etc. It's very easy and lots of fun. Use very little paint as you don't want "solid" color. After you put a little paint on your sponge, kinda dry it off on paper towels. Also, use your hair dryer to dry each color before using the next color. If this sounds good to you, please try it on a scrap piece of muslin first so that if you make any goofs you won't ruin your green bag! Just an idea.......