Board Thread:Questions and Answers/@comment-34681548-20180213052442/@comment-2182964-20180213161815

Looking at the examples in the series, it can be inferred that it boils down to the spiritual energy of the user. Ulquiorra has a green-coloured Cero, and his reiatsu is similar during his release, only much darker in colouration. Yammy predominately shows a red-coloured Cero (and Bala) and has a red aura when released, for example.

Though this is hardly what I'd consider 100% accurate either. Grimmjow's spiritual energy is blue yet his standard Cero is red. Gran Rey is blue though, so there's that. Because of this I'd lean more towards what Yyp says, because aside from speculation it wasn't addressed in the manga or anime.