Starting from Doomsday 1.5.5 it's possible to adjust gamma correction, display contrast and brightness using console variables. The requirement is that your display adapter supports hardware gamma ramps.
The console variables are:
Gamma correction factor. 1.0 is the default, which results in no correction. Values greater than 1.0 increase display brightness. The smallest value you can use is 0.1.
Display contrast. 1.0 is the normal contrast level. If you increase contrast it's a good idea to increase vid-bright as well.
Display brightness. 0.0 is the normal brightness. Positive values increase brightness, negative values decrease it. Good values are pretty small: if you for example set vid-contrast to 1.2 it might be good to set vid-bright to 0.1.
When using these variables make sure the software gamma correction is turned off: setgamma 0.
If your display adapter doesn't support hardware gamma ramps or you're using an old version of the engine, you might try the following:
The PLAYPAL lump consists of 14 palettes each 256 colors long. Doomsday only uses the first palette (the first 3*256 bytes) but it's best to operate on all of them.
Extract PLAYPAL to a file using the -dumplump option or a simple WAD extractor (Wintex for example).
Open the file with an image editor such as Paint Shop Pro or Adobe Photoshop as a raw image. I'm sure other programs support the raw format as well. You're most likely prompted for the format of the image (raw images are just pixel data): set width to 256 and height to 14 (as interleaved RGB). You should get a nice picture of the palettes, each row being a separate one.
Now tune the image with gamma correction, brightness, contrast, etc.
When you're finished, save the palette image back to the file and load it into the game.
You don't need to create a WAD file out of the modified palette lump, the engine can read single data lumps as well. You can use the -file command line option, or put the new palette file to your defaultWads in the .cfg file. You can also load the palette at runtime (with the “load” console command) but it won't necessarily have an effect everywhere that way. Certainly not before forcing a texture reload.