The Witchwood Forest (Ars Magica 2) has the right colouring and does generate, but is populated with oaktrees instead of witchwood trees (I'm quite sure it isn't a biome id conflict as I don't even have BoP, ExBXL or EB installed currently).

http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/wip-mods/2524489-wip-realistic-terrain-generation-0-0-17?comment=539

Ars Magica 2 issue: Mithion/ArsMagica2#1284 Nothing todo with RTG I guess.

Thanks for letting me know @Adaptivity - much appreciated.