Camp Forlorn hope, the Dam, Caesar's camp... all bathed in a strange red/sepia tone.

Anybody know the significance of this?

It's a desert...

its the time of day. at least for me evening has a weird sepia tone in some locations

I don't think it's sepia. The color scheme over those places is much more red, I think it's because of the atmosphere, listen to the background music in those places. Especially in Nipton, both combined generate heavy and menacing atmosphere.

Have you been to those locations?

The colour tone in them is very different from the rest of the desert in the game.

Perhaps Sepia isn't the word... it's kind of a reddish brown colour. But it's unique to that area around the river and very different to the rest of the map.

The scientific explanation is that "All the other colors got blowed up in the war."

Seriously, though, I doubt the game engine limits color use. I usually assume programmers know what they are doing, and crap like this is an "artistic decision" by the white loafers. How do you make something depressing? Turns out it's really, really easy: just limit the palette to subtle variations on a sort of puke grey.

It is a design choice, but it's too apparent to be a good one IMO. I got headache on the eastern fringes around the Nelson area. lol Everything gets bleached of colour, s'too uh, much.

As much as you say it is the overall colour palette of the buildings, the user interface being orange will accentuate this. You could change it and see if this helps at all.

Reno