I would go with red, personally. If you were painting green, it would be a little tough to make it stand out... I had the same issue with my Black Knights for fantasy, I made a banner carrier with a head impaled on the top, and painted the banner red, then tried to paint blood on it... I was able to make it look passable, but I should have thought about it before hand and made the banner yellow or orange or something like that...
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Armies: Ultramarine, Inquisition/IG, 40k Ork, WFB Vampire Counts Skeletons
Orks:
3 Wins
3 Loss (tweaked list did NOT work)
IG - Daemonhunters:
3 Wins (Another tourney win against Necrons)
14 Losses (tourney losses to IG and Nids)
3 Ties
Vampire Counts 0-2-0 (Got slaughtered by Karl Frantz)