hated those fucking desert tiles from Day 1..but, according to people that been there, YES it is that bright.
the only thing I can think of, is write a terrain specific EnvrionmentalSystem.ini, and try to fiddle with sun levels (which WOULD be very bright in the region).
or, if one is reaaaaallly good with photoshop, try changing the brightness on each and every desert and desert2*** tiles -- somewhere in the neighborhood of 40 or 50 tiles?? including all transitions???
or, again, if one is really REALLY good with PS, create fully new tiles, using the IsrealME desert as the base, and recreate all desert and desert2*** transitions; again, an exceeding large job. Done with layers -- IME desert on the bottom, 'tile to be fixed' on top, and gently follow the transition "parting line" with a very small eraser tool, and remove the original desert section. Gradually moving up to larger erasers until all the original desert is removed. (btw, IME tiles truly suck in their skewed angling)
have fun!!!