Historically, gamma per platform
Mac, 1.8
PC, 2.2
As far as I know no add-on install can change your xplane gamma setting. Take a look in your computer os settings for the display, if you can see it, it likely will report 2.2 (pc). At some point within the last 6-8 months, X-Plane default pc gamma went up to 2.5, which in my opinion and logic is 'over standard' and done for the purpose of making things look brighter. What it ends up doing is washing out colors and distorting shades that were designed 'at standard' values. You can make your display look brighter by jacking up the gamma, but reds turn pink, blacks turn grey, blues turn aqua/light-blue.. etc.
I suggest, setting the computer side gamma settings back to a 'standard' value. X-Plane default may be 2.5 for pc's, but 2.2 is 'standard', and 2.0 will give things more depth. Mac standard is 1.8. PC's rarely if ever color calibrate screens, just go with whatever it shows when you turn it on.
I suggest, pc users, gamma 2.2. the whites stay the same, the colors will get deeper and richer. same will be true at 2.0 but then you'll be on the deeper & richer side of things and some things may appear too dark. But right off the bat, pc users should change their x-plane gamma to 2.2. and for "brightness", most monitors and flat panel displays have a gamma control in them as well, which can goose up the overall brightness. My Sony display has gamma-1, gamma-2 and gamma-3. anyway. keep this in mind. properly calibrated colors, black = black, white = white, and the full range of color happens in between. when blacks are grey... somethings off and all colors are not what they're intended to be. adjust accordingly
I 'want' to add, that most computers should have in the os prefs for displays and color, some form of color calibration. Usually it's a series of things on screen that you adjust till what you see matches what it's supposed to be. Different things may do this differently. because what you see, is a product of many layers of things generating images based on what they think the color space is. there's the os level, there's the application level, then there's the display itself. They dont have to match, but when they're all doing something different.. chaos .. visual chaos can happen. The net result being what you see, should be calibrated, and then the application itself should not modify that. OS and Display are the two constants. I also want to add that in most places and cases where there is a color calibration faculty, there are color profiles avaialble already, cycle through them to see what you see. white should be 'paper' white not 'blue white'. black should be without any color dark black hole "black". there's other stuff about the temperature of the white, 6500 being os side paper-white, while most monitors want 9300 for a white-white. anyway. based on my years in graphic design and magazine publishing support for color-space, color sync, color calibrations.. I know my setup is calibrated, and i'm reasonably sure that for pc users, set the application gama to 2.2 and things should look 'normal', or as normal as they can if everything is consistent #'s. 2.5 is just too high (ty xp default value)
http://en.wikipedia.org/wiki/Gamma_correction
http://www.photoscientia.co.uk/Gamma.htm
(the photoscientia link is cool. at least fun, I recommend)