Personally, I think the consistent colors suck. Too boring for me. More color the better. But to answer your question, a color change for a tile has to be pushed from a server. So a dev can not offer color options from within their app, unless thy already have a push server setup, as with live tile apps, to push the change. Smart Tile is one of very few apps (possibly the only) that offers about a dozen color options for their tile which they push from their server. I wish more developers (I have contacted all the transparent guys) would offer color options, or at least offer transparent or their default tile. With the ability in Mango to update apps without the need for a server hopefully there will be more options.
Nothing personal, but you are the reason 28 of my 46 tiles are the same color. And more changing/updating every week. Why don't we jus get rid of the colorful tiles completely and list everything as text, white letters on black background, or vice-versa. And maybe the weather app could match the accent too, so we cm have green clouds on a green background. In fact, just make the text and backgrounds all the same color, then you won't have to deal with anything non-compliant.
Sorry for the rant, but the WP Start menu is graphically exciting to look at with a variety of tiles displaying images, live updates, and everything else. Some updatable times with small font text look better, white on black, rather tan white on orange or pink. So there is a practical reason as well as aesthetic one for having non-conforming tiles. Can't wait for Mango and tiles that will flip, showing breaking news images, headline updates, messages, etc., and hopefully not all the same color. Don't want to live in a boring DOS world.