I feel like they would look best if they followed the more of the original designs and used the colors of the decepticon Headmasters; with the last one being in horrorcon colors since both kinda share the same colors.
im not the biggest fan of sg decos just being rival swaps. i like the ones that dig a little deeper in the character's history.
as i said above, if i were coming up with these decos myself:
Hardhead would be wearing BW Neo hardhead's colours due to the shared name.
Highbrow i'd give the colours of HFTD Highbrow
Chromedome might wind up in Arcee's Pink/white as a nod to the g1 headmaster arcee concept as a chromedome repaint
Brainstorm would be Orange with some black and green striping as an homage to the Ben Ten alien of the same name.