It's a feature. Sort of.
The Honey Badger has a very distinctive colour scheme in the physical world because it is not at all painted, it's made of several different alloys of aluminium, and those sort of cream and grey shades are the natural colours of those alloys.
By default the game gives you the weapon with a paintjob that seeks to emulate the natural colour of a physical Honey Badger's receiver. Of course the game gun's receiver is still actually black undernearth as you can see from the default worn version of the paintjob that lets a bit of black be seen.
As I mentioned last month I was fairly sure already---because of the default image of the weapon we could see in weaponsmith before we could actually access it---that the tan colour would be repaintable while the grey handguard would not. And here we are.
Call it Ubi's attempt to reference the natural look of the weapon.
Of course
logically if you can repaint the receiver there's no good reason you can't repaint the handguard...except that weaponsmith does not support multiple paints on a receiver mesh.
Remeshing the handguard with the barrel would be acceptable to me since the barrel can't be seen anyway, but obviously they've chosen not to do that.