This is going back quite a way, but I think the uniform Nicholas is wearing in post 131 is from the Royal Scots Greys, and taken during the visit to Balmoral in 1896. Bear in mind that, then as now, a regiment has several different 'orders of dress' and they may well look quite different. The Greys uniform he is wearing in the Serov portrait is full dress (which, with the bearskin) looks very much like that worn by the Foot Guards).
I don't think it can have been taken in 1890 because Nicholas didn't have close links with Britain then, and I think he had yet to grow a beard and still just had a moustache.
Ann