One of my big bugbears with the old skin system in general is that your skins did nothing for the attachments/mags, which I tolerated but was mildly annoying. Now that's no longer the case - and that's great! I love it! But now I find that if i'm using, say, the old twitch rivals skin with the cool crystals coming out of it, it's back to the old default-color attachments, and it kind of disincentivizes using those cooler, custom-model skins.
This might just be because those skins are older and a relic of the old system, and newer model skins will change the attachments as well, but I think letting them select a secondary color would be a good compromise.