You won't get the exact effect, but good possibility is brush embroidery - color the fondant green with marbling let it harden, pipe the piece sign with thinned royal icing and paint it with damp brush.
You have to make it in small steps - the royal has to have very thin consistency for painting.
You can paint with your food colors and water, just like you would paints. Just use a nice large brush for your green - I'd actually use two greens... maybe a kelly and a leaf green. Put a dab of color on a styro plate, add some water and mix. I probably wouldn't mix it really thoroughly because it kinda has streaks of color in it. Use your large brush and brush it on in large strokes from side to side letting it be streaky. Have a damp towel handy in case you need to soak up any excess. Once you've done the entire background, go back with your black. Don't thin it very much. I'd paint on the peace sign and then go back with just a wet brush. You could actually drop drops of water on the black and then pull the color out and some in. Be careful not to use too much water because while you want the look of water colors, you don't want it spreading beyond control. You could use airbrush colors as well and they won't require watering down. You could also thin it with an alcohol, but I've found it's not necessary for this look. It might just need to sit overnight before it's dry. Good luck! Hope it made sense!
I paint on fondant all the time with gel colours mixed with a bit of vodka - which evaporates faster than water, thereby not making the fondant gooey.
Thanks for the suggestions. I really appreciate it!