If you embossed a design into your armor, simply draw over it with fabric paint from a squirt-tube to create a raised design. You might have to do this more than once to make it more visible. Since the results will be thick, allow this to dry overnight. Since the foam is spongy, you will need to seal it before applying any glue. One suggested mixture is 1 part school glue or Sobo glue, 1 part flexible fabric glue, and 2 parts water. Apply and dry thin coats until sealant no longer forms holes where air bubbles from the foam pop through. This may take as many as 7 or 8 coats, but because the layers are thin, the dry time shouldn’t be unbearable. Don’t allow debris to stick to the glue or it will produce bumps in the armor. If the armor sticks out in places (leaving the underside exposed), painting the back will give it a more professional look. Because the foam will bend and move with your body, ordinary paints will crack. On a piece of scrap foam, experiment with flexible craft paints (ex. fabric paint) to see what will work for your design. Be sure to apply the paint evenly to prevent streaking and work it into and cracks or crevices. This can be done by brushing dark acrylic paint (ex. a mixture of black and green for a tarnished copper look) over your armor and wiping most of it away before it dries so that hints of it remain in the cracks.
Apply raised designs if necessary. Seal the foam. Paint over the back of the armor with acrylic paint if necessary. Paint the front of the armor. Give the armor a weathered look.