Is there a way to make my Mossberg 500 camo?

Simple spray paint is the cheapiest way to go and you can get stencils to make all sorts of neat patterns. When it scratches or you decide to change patterns its just back to the stencils and paint.

The dip processes run around $150 and up.

I agree, spray paint is the simplest and cheapest way to go. Rustolium and Krylon both make flat camo colors for just such an occasion and you get a big can instead of those little camo kits.

Do you really need camo? How about just a flat OD? Im kinda thinking that turkeys arent really going to notice the difference.

This isn't normally about "need" its more about want. You want to put evrything in your favour to use every option at your disposal.
Turkeys have been shot with blued barrels and wood stocks for years, however the addition of one more thing to help break up your outline and the outline of your gear when chasing one of the sharpest eyes in the woods is just an added advantage.

Spray paint and just pull off a branch with leaves to use for a stencil. It'll match the area you are in. No need to waste time and money making stencils.