Honestly spray paints would be the cheapest and a Digital Camo pattern Wouldn't be hard at all to do. i found a good plastics spray paint called Krylon Fusion for Plastics.
A simple digital pattern would be easy to make with some card stock and spray paint. Here's a good way to go about it.
1. Remove all peripherals from the mask (i.e. goggles, straps, visor, etc.)
2. Spray a base color, preferably olive drab all over the mask and let dry.
3. In the mean time cut a random pattern of rectangles out of a sheet of card stock and attach it to the mask and spray a different color like black, brown, or another shade of green. Let dry.
4. Use the same pattern, but rotate it 90, 180, or 270 degrees and spray over it in a different color other than the color than the previous step. Let Dry.
5. Repeat step 4 as desired.
6. If you want to spray the visor due so using steps 3-5.
7. After the mask sets ling enough to dry then put it back together and enjoy.
NOTE: Follow paint instructions listed on paint container for safety precausions and painting instuctions such as paint drying time.