using programs that alter graphics is against TOS and can get you banned.

TOS against mods is so we can ban cheaters. I agree DarkD3 is not a cheat...yet. As long as it is not a cheat, no ban is likely.

Here's a video showing Diablo 3 with a darker color scheme. It works as a direct3d post processing filter, making the game look sharper, have less saturation, add a slight bit of a (silver) sepia tone, slightly lowered exposure, slightly raised gamma.

[NEW - WEBSITE & DOWNLOAD]
http://darkd3.com
Preview with screenshots, compare and download all the different effect presets in one place.
Downloads are now only avaliable from the http://darkd3.com site, due to unreliable file hosting sites.

Disclaimer:
What you are about to download is a Direct3D hooking filter. It is a DLL file and some shader filter files that you place in the game's install directory. IT DOES NOT MODIFY ANY GAME FILE OR PROCESS MEMORY. The Steam Overlay and the FRAPS fps indicator use a similar method to change what directx and your video card renders on screen. BUT still I'm not 100% sure if something like this is allowed by Blizzard or whether they will count it as a "mod". So use it at your own risk, even though I've used a filter similar like this for WoW for a year, it was called ENBSeries, some others used that and never heard of anyone getting banned for using it.

Current issues:
-Running FRAPs, or launching the game with Steam Overlay makes the filter possibly not work on NVIDIA cards.
-The game seems to crash with the "d3d9.dll" file in the folder the FIRST time you launch the game or after a patch update. To resolve this, rename "d3d9.dll" into something else, then launch the game, close the game after, then rename the dll file back to "d3d9.dll". Then to launch the game make sure you run "Diablo III.exe"? and NOT the beta launcher (which also happens to be the link in your desktop).

Instructions:

1. Visit http://darkd3.com select and preview the effect you wish to download.

2. Extract the contents of the downloaded zip into the root directory of your Diablo 3 install.

3. Start the game and turn off/on the effect using the Pause key on your keyboard.

4. If it doesn't work read the current issues list at top.

Credit: I take no credit for the actual coding and creation of the DLL file and shader filters, I based this on this mod for skyrim: http://www.skyrimnexus.com/downloads/file.php?id=131
The only thing I did was tweak the filter settings to match what you see in the video.

Tweak it yourself:
If you know a little bit of coding and syntax you can open up the file darkd3_Settings.h in notepad and edit the filter values yourself.