I think it's important that the colors of the environment, at least exteriors, be muted and drab where they exist at all. Part of this is to emphasize the depressing and downtrodden nature of the Final Empire, and part of this is to emphasize clean colors as the mark of wealth and power (white especially, since it's impossible to keep clean. Both the Lord Ruler and Elend wear white uniforms).
I'll go with the sky being amber/yellow/orange, when it's not obscured by clouds. Lately I imagine it as if the sky is always in sunset colors, even at noon. Wherever there are plants, whether trees or grass or crops, they're a shade of brown (Sanderson is very clear on this). Plants are generally shriveled and unhealthy at best.
Luthadel is defined by it's walls, and it's packed right up against 'em. And since you can't build out, you build up. Nothing should be less than three or four stories tall, unless it's in wealthy territory. Rooftops are either slate, tile or wood, and usually rise to a peak. Buildings are described as mostly stone, and I expect what he means is block-and-mortar and brick.
I imagine the architecture of Luthadel to be a blend of London and New York during the Victorian period. I agree with the pseudo-Victorian styling for the mansions and palaces of the nobiliy, but I envision the skaa slums as being more like tenement housing, rickety and tall and very poorly built/maintained. And of course, somewhere in between is the housing for the middle-class, the skaa merchants and industrial workers who, while still skaa, live about as good a life as any skaa in the Final Empire of the Lord Ruler can expect. Probably lots of that false-front faux gingerbread styling.
As to the colors of the city, almost everything is shades of black, stained by centuries of ashfall. He's quite clear about the ash stains, saying that "every structure in Luthadel... had been blackened to some degree... structures were generally darkest at the top... like paint running down a canvas, the darkness seemed to creep down the sides of buildings in an uneven gradient (p28-29 of the first book, which has a generally good description of Luthadel as a whole). The streets are solid ash-black, whether they're paved or not.
Don't forget that parts of the city are given over to industrial manufacturing like textile and steel mills, which come with smokestacks and drains and other sources of pollution. The Lord Ruler suppresses some technologies (like guns) but allows others (like canneries) to flourish. There's also a Hotel District and a Commercial District, each of which would probably mix a variety of style and quality of architecture to suit the business of the wealthy and common alike.
slums and tenement housing:
http://www.cartoonstock.com/lowres/csl0490l.jpghttp://www.theglasgowstory.com/images/TGSE00850_m.jpghttp://newdeal.feri.org/images/d45b.gifhttp://www.ngaiopress.com/slum_big.jpgImagine this skyline without the trees or billboards, all stained black:
http://geneastory.com/global/page5/assets/GlobalSemester276%201.jpg
