You can probably create your own from masking tape. I was just remembering the hex pattern from the HUSH tutorial. You could do something similar, but by cutting irregular shapes. It's more labor intensive, but I think it'd work with a regular brush too. Of course there's no need to use this approach at all, I'd probably just paint it like normal without a stencil.
As for the colors, that sounds good. I think the important part is to get the color variation in there. Lighter on the belly and darker on the spine is a good scheme. Spots, stripes, etc can be interesting, though I would keep them subtle and not over do it. Check out images of real lizard for more ideas. You could even go for more of a red or purple main color (could be nice next to the bronze armor), though nothing wrong with green if that's what you prefer. Consider the armor and what colors you plan for the rider, then pick what you think would work well with all of that.