The tool to use, once you have a series in basic colours like green, brown, black, tan etc etc and have applied the palette and are happy with how the basic single colour Icons turned out all you need to do then is use the
freehand tool to carve a camo pattern out of one base colour then cut and paste it to the base colour Icon. You need to line up the slice exactly with the base icon but that is easy once you get the hang of it. If you want a three tone pattern then carve the colour out of the new one then past it into the base. The trick is knowing what combination of lightness and contrast to use when lightening or darkening the basic Icon to generate the coloured ones becasue each one needs its own specific TLC before the pallete is applied.
These were just done quick as an example then enlarged 2X. Sometimes you need to darken or lighten areas if needed
Don
The image has been reposted in PNG format