Two things. The Jhereg colours are black and grey, not just grey. And wearing clothing entirely in your house colours is not required. The impression I've gotten is that if you want to indicate your house by clothing, you just need to show enough of the colours to get the point across, though going all out isn't unusual. And if you don't feel like wearing your house colours, you don't have to. Daro for instance got away with dressing in Lyorn colours, to many people's confusion. There are also plenty of examples in the books of guessing at house from facial features, which would imply that the person being scrutinized is not wearing identifiable house colours.

Otherwise, cool picture.