There are some people who believe in "dark" and "light" keys. I am not one of them (or, at least, I freely admit that my hearing, as developed as it is, is not "advanced' enough for the concept to make any difference to my ears).
With instruments and keys, you should be worried about range. In other words, most instruments, other than piano or organ/keyboards, do not sound as good across the entire sound spectrum, and playing melodies on certain instruments in certain keys might be stymied by that particular instruments range, or total amount of useable notes available. The art of orchestration/arranging is often one of wise use of the proper instrument in the proper register/range. Same goes for people's voices-- we can't all sing great in all ranges from bass to soprano. So you've got to choose instruments that fit with what you want to do in a particular key (or vice-versa, pick a key or keys that make your featured instruments sound good).
As far as "ambient/late night/hazy/deep feeling" sounds-- these are what we call "producer words." You need to define them a bit before anyone can give you an answer that makes sense...
GJ