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Red, so it goes fasta!
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There's no one answer here. "Orkifying" can be done in many different ways. Here's a few points to consider though
1)What Clan do you play as? That's a good place to start. (Deffskulls would paint it blue, Evil Sunz and Speed Freaks-Red, Snakebites- a lot of animal horns and totems)
2) If it was recently captured, then it might have its old Imperial color scheme. What Chapter did the Rhino belong to? Consider using some Marine Transfers as well as adding some Orky ones. You might want to add battle damage as well (the rokkitt or PK hit that brought it down in the first place?)
3) If you want to add the "Red Paint" job upgrade or not.
"Orkifying" is one of my favorite reasons for playing an Ork army. There's no right or wrong way to do this. Remember that "eks-pirimentin'" is an honorable part of Orky Kultur.
My award winning Orky tale