-Create a backup of the BG .map file. You may need it, you may not. But make one anyway.
-Open up BG in HMT, then expand bitmaps. Scroll down until you find "levels/test/bloodgulch/bloodgulch".
-There should be several bitmaps under that category, listed out on the right side of HMT. They should be in the format "R5G6B5".
-Create a folder at a location of your choice, then extract ALL of these textures to that folder.
-Now, I'm assuming that since you're used to editing bitmaps, that you have photoshop and the correct .DDS plug-in. Now, go into Photoshop, and open up one of the R5G6B5 textures you just exported. If asked to display mipmaps or not, choose no.
-You will notice an actual textured area and the rest is a yellow and red checkerboard pattern.
-For this to work right and not make BG look like a freaky tripped-out yellow-and-red level, you need to select ONLY the textured part of the file; the checkerboard pattern should not be edited. My guess is HALO has it hard-coded into the game to not display those hues of yellow and red, b/c if you darken them as well, BG shows up all yellow and red in a lot of spots. I used the geometric lasso tool (or whatever it is) for this, since all the edges are square. It took a while, but I think it produces the best results as opposed to other selection options in this matter.
-Ok, so now that only the textured part is selected, darken at will!!! I suggest using Photoshop's brightness slider, so that the different colors of the bitmap are preserved, instead of just painting over it in black. You may have to exit and re-enter the brightness slider several times to get it to a darkness of your liking.
-Now, REMEMBER the total percentage you darkened it. (for example, if you moved the brightness slider to -100% 3 times, the total would be -300%. If you then go back and make it -49% darker, the total would be -349%. Get it?) Trust me, knowing this will allow you the best base for final tweaking of the darkened look.
-Ok, Save your changes. Except, click "Save as..." insead of "Save" When it prompts you for a location, filetype, etc. , put it into a location you'll remember, and put the number that was in the original file's name at the end so you know which one to inject it over later. Be sure to have ".dds" selected as the format. Hit "Save".
-With the .DDS plugin, you should now be prompted with a menu with a whole slew of choices. Select "16 bit [5:6:5]" as the format. For the mipmaps option right below that, choose "Use existing mipmaps". Click "Save" again.
-Congrats! You just edited part of the lighting of BG. Now repeat for the other R5G6B5 textures you extracted, using the same total darkening percentage that I told you to remember earlier.
-Reinject all of these edited ones over the originals using HMT. Be sure to inject the right textures over the right numbers...this is why I told you to keep the original number in your edited texture's name.
-For some reason, when editing and re-injecting some of these textures, when you load up the level, some weird shit is displayed at points, but only sometimes depending on the specific light textures edited....to fix this if it happens, just write over the BG .map file that was in your MAPS folder at the time of modding with the backup I told you to create earlier. You shouldn't need too, but including this step just incase.
YAY! You have now darkened BG successfully. Load up the level and see how it looks. To make it look right, you will have to make the sky night-themed as well(I just injected the sky textures from the level where you start out sniping covies...don't currently remember the name of it). Go back and darken/lighten all of them until the level is darkened to your liking. That's it, enjoy! Hope this helps all of you out. You can use the same steps for any level (to my knowledge), so this is a universal tutorial (i think). Again, have fun with this, and enjoy!
