Here is a Kolossus I have painted to be used as a medium/heavy tank for my 15mm Lhurrg forces. In 5150: Star Army, walkers simply use the same rules as tracked vehicles. I prefer walkers as they are a bit more suited my 2x2 play area.
It is painted in the same colour as the Lhurrg skin, to help tie it to that force. I experimented with a chipped and rusty painting technique to show that the Lhurrg bought this walker and repainted it. Then didn't take very good care of it.. It's meant to look like it's been in the field (and out on the rain) for a long time.
To sum it up, I panted the whole model dark brown and gave it a wash of brown/black mix. I then stippled on the yellow paint, avoiding the edges where the paint is supposed to look worn off or rusted through. Once that was done I did some very simple highlights and used some sepia wash in the joints and deep crevices to help it look rusty.
The black markings are all freehand, which was something I havent done in over a decade. The white kill crosses are deliberately more crude.