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So let me see if I understand this right... Take a piece of bamboo, hold it up vertically and at various slight angles, and spray paint up it with say black and light brown?

Generally you will want flat natural colors. If you are spraying do the following but if you are using brush on paint then paint the actual stencil and stamp the wet paint covered vegetation on a previously randomly camo painted background.

With spray paint and real life stencils you are effectively painting the shadows behind the grass or leaves you want to represent.

Pick the color you want the grass stems/leaves to be. Spray an area that color. Or better yet use two slightly different colors of tan, or light yellow, or green in a random pattern. let this dry.

Then get a wad of what ever grass, bamboo, leaf pattern you want on the boat. fan it out so it loosely covers the area you just painted. Now use a darker spray than before and over spray the fanned out leaves while keeping them as still as possible. Try to only spray perpendicular to the surface of the boat so the paint does not go under the leaves. In some areas spray it a little darker an in others a little lighter. For a more realistic look you can then move the fanned out leaves VERY slightly (only 1-5 mm and lightly spray again with the shadow color. This will add natural shadowing and highlights giving the leaves or blades of grass more depth of appearance.

For even more added realism you can then add some very fine splattering of black, brown, purple, red, or bright yellow based on the colors your target vegetation turns as it decays. Think about all the little specks of color on the blades of grass when you are searching for blood droplets tracking a deer.

As for the suggestion to not use black. When spraying on paint the underlying color shows through with light layers of spray and thus black is really darkly tinting the underlying color until it get thick enough to actually look totally black. Black also provides the best contrast with lighter colors and thus produces more depth or 3d like appearance to the pattern when used correctly.