Yeah, but would he recognise it, based off that...

I guess in my head, this stuff is like IDing a not particularly potent ingredient in a sauce, rather than recognising a colour. I guess it's down to how good he is at this stuff.

He'd definitely be able to tell, if he had any dark water or dark water-touched items near him, for a decent reference point.

It has a similar vibe to the heart Kaneki has been carrying around, if that helps.