Group images in 9 color sets with matching forum icon sprite (including MyBB default).  Choose one color or mix and match.  Base colors below if you want to coordinate your theme.

Blue: #00AAFF     Red:  #FF0000     Green: #019E19     Yellow: #FFCC00     Orange: #FF8000     Purple: #8000FF     Pink: #FF3399     Grey: #8C8C8C