Even in sourceports fixing the blood colors (like Crispy Doom making Hell Nobles' blood green, Cacodemons' blue, Spectres' transparent, and Lost Souls' bullet puffs) the Demon Lord's blood stays red as he's a DeHackEd-modified Mancubus. So I asked antares031 from Doomworld forums to make the blood on the dying Demon Lord's body red. Enjoy!
Personally I prefer the Hell Knights and Barons blood to be red.
Do you use modified corpses as well?
The blood color fix in Crispy Doom is optional.