Yeah, in the latest update milestone add some carbon layer in shark stock helmet. And from my knowledge, yes it is related to clearcoat_mask and material_instance made by Milestone. Thats why some helmet can have carbon layer effect and some don't even though you changed clearcoat_mask to purple. Shark stock helmet now have its own clearcoat_mask texture in clearcoat000_mask. So in order to remove the carbon effect make sure you edited the correct one.
View attachment 390126
The way milestone add carbon effect as per my understanding is adding additional normal map (T_CarbonFiber_N) into the object based on the mask which is on blue channel of clearcoat_mask texture.
View attachment 390127
You can remove them by changing the blue channel to plain black and you literally will only get red color.
My conclusion based on my analysis of clearcoat_mask are :
Red channel = clearcoat layer mask
Green channel = roughness coat layer mask
Blue channel = carbon fiber layer mask
Black on all channel mean you get no coating layer at all, the combination of mask in per channel can provide variaton of coating effects such as, yellow = r+g you'll get rough coating, purple = r+b you'll get clearcoat + carbon fiber and, white/close to white = r+g+b you'll get rough coating + carbon fiber.
And I need to reiterate again that not all helmets can have the same effect, it depends on material_instance configuration made by milestone. Some only have config for clearcoat (red), some have r&g channel config, and some of the others have config for all rgb channel which you can found in object that have carbon fiber effect.
Sorry if I'm being too long lol, hope you understands my explanation.