(Well well, I've outdone myself - this is one long post!)
Ahh, well, with such a short list of colors, I understand the problem.
Black - unusable for monsters :/
As a player of LDC, I've learned 2 things: one is that shadows (black spot) work just fine, and the other is that mimics could make for a great thesis in psychology: (I assume you've played it, but just in case, and for others) in UI terms they act just like any other monster, so that when you 'x'-look at them and '?'-get details on them they'll give one of several amusing monster descriptions, but since the icon used is the same as that of some dungeon item, be it cash, consumables, normal equipment or an artifact, they'll fool you almost every single time!
You're dealing with human players, and only a real pyscho'll bother to check every single tile for tricks - and since this game's ment, the way I see it, to play on that human's feeling of security, encourage him to turn into that psycho! Let him fear the red dots (I'll explain in the monster thread)! Of course, this is only good to the point that keeps the game played, but on the other hand, these kinds of things might only make it better.
Also NetHack's Stormbringer's a good example - it doesn't give warning on the attack of a friendly. Mostly that's harmless, until you screw-up.
Cyan - too light for our mood
LightRed - too light for our mood
LightCyan - too light for our mood
LightBlue - too light for our mood
LightMagenta - too light for our mood
LightGreen - too light for our mood
Yellow - too light for our mood
Not if you're going psychodelic like I had suggested. In that case, these colors'll get to people for the same reason clowns are such a major focus of phobias.
But, as I had said, the player'll first have to allready be in a dark mood, which will be created throughout the early-, mid- ,and possibly as far as late- game by using a "normal" dark environemt.
That leaves us with
Blue
DarkGray
Magenta
Brown
And together with at least 52 characters, that's enough (if not perfect, as more may be too much) for a dark, gloomy environment.
Especialy that the more dangerous monsters should somehow be "highlited" for players convieniece sake...
"Screw the players" :) This game's supposed to be good, that's obvious, but it can be good because it's balanced and comfortable, or it can be good because it accesses a player's every inch of personal space for an immersive sense of panic and fear combined with the character's ability to deal with that fear if he would just try - that's the ultimate fantasy for a dreamer, gamer or book-reader: that whatever tool they're using to expand their world will not be so timid as to say "sorry, I didn't want to surprise you".
Indeed, I might be missing something terribly, and people just want a complex puzzle - that's what's great about NetHack - but I really think that since the unique upside of DoomRL seems to be a dark atmosphere with growls and bloodstains, taking several steps in that direction can only help.
And, above all, this game isn't real-time, so very few things can't be thought through effectively. In the event that you find yourself in a terrible position that could be changed if you were calmer (like checking the nearby tiles), you can always take a break, just like you can always close the book (just as they can read the end and go "ha, it didn't scare me one bit!", savescumming et al :) ). But the fact is that most keep reading when shit gets rough ^^
All this, BTW, opens a very interesting option: a challenge based on turning off the 'l'ook mode.
So yeah, I tend to agree with Adral's post, though I think stealth-warnings are only good if the monster may outright destroy you otherwise and/or if stealth is UI-independent and constant, and friendly characters should be in the mood of that char, which is likely low as its home's being invaded by demons, so dark is still good. Being a '@' should be enough. It could then even revert to the char's race icon when in berserk mode to signify friend-foe blindness.