Quickest, easiest custom ever. Took all of ten minutes. I don't usually deface the only copy I have of a figure the exact same day that I buy it, but from the moment the Worlds Collide set was revealed, I felt like the colors on Nemesis Primal were gorgeous, but lacked a certain je ne sais qouis in beast mode. I suspected a silver dry brush would make the gorilla mode more distinct, work as a "silverback", and compliment the bit colors well. Still working on Thunder Clash, but this was low hanging fruit and I feel like HasTak should have done this or something like it in the first place.
It's super-easy, you should give it a shot. Like I said, it took ten minutes, and it was dry enough for me to take it outside and photograph maybe ten minutes after that. So what you do is you get your acrylic paint (I used Tamiya X-11 Silver because Tamiya is rad), you dip the tip of your brush in the paint, then you wipe the brush on a paper towel until no more paint comes off. Once you've done that, very lightly flick the brush over the fur. What you're trying to do is get the paint to stick to the parts that are higher up, while leaving the recesses with no paint in them. (EDIT: Acrylic is water-soluble, so if you make a mistake, you can just wipe it off with water if the paint is still wet. If it's dry (or if a few weeks down the line you decide you don't like it) soak it in a bowl with some water and OxyClean and the paint will come right off.
Yeah, transformed him into beast mode first, since I didn't want to accidentally hit any of the 'bot parts. I'm sure the set is going to be a pain to get, especially if you happen to be in the UK. Target exclusives are BS.