Colouring isn't a style... You can have a load of colouring techniques to differentiate styles. I'm not saying everyone should draw exactly the same or anything - just colour it in.
Hell, I don't particularly like colouring at all, which is why I tend to stick with flat colours or just lines. But if I'm doing a collab piece, especially one with a generous timeframe such as this, then there's no excuse for not putting a bit of colour on. It doesn't take THAT long.