World of Warcraft's 6.1 patch will add a more robust suite of options for colour blind players to use while in Azeroth. Blizzard have had a colourblind mode in World of Warcraft [official site] for a few years now but the next patch will bring a new set of filter and strength options to better cater for people with colour vision deficiencies.
The mode as it was previously did things like adding text so players could tell the rarity of their loot without relying on the colour system. There were also console commands which could be used in order to alter the colour palette of the game to correspond to different types of colour blindness which players then used to help make the game interface easier to deal with.
I don't think that was built as an official mode, exactly – it was actually a way for the dev team to simulate colour blindness for testing purposes that was then useful to people with forms of colour blindness. You can see the effects of various shaders through the links here.
This explanation by forum poster Neranil explains the benefit of the simulator as opposed to the previous official move:
"The idea of a colorblind shader is great because it can boost the colors you're weak in or shift colors toward ranges that are more perceptible, increasing differentiation and making the colors identifiable. A separate mode has to exist for each major family of colorblindness, because of the variations in "safe" ranges to use. In some ways, it's better than the "colorblind mode" in the options screen, which just adds extra text you can read. I'm really happy to see this sort of technology appearing in games and Blizzard has been leading the charge here."
It looks like the version Blizzard will be releasing as part of the 6.1 patch mixes both previous approaches. Loot rarity and other colour cue-dependent qualities are given text augmentation in their description. As the official blog explains in an example, rare gear whose name is traditionally listed in blue gets the word "Rare" added.
In terms of the colour filters, these have been designed to help people with protanomaly, deuteranomaly and tritanomaly. The first two are types of red-green colour blindness and the third relates to blue-yellow. To activate them you need to select whichever one you require from a dropdown menu. You can then adjust the strength of the filter by using a slider: "The farther to the right you set the slider, the more the game removes potentially problematic wavelengths of color for each type."
As Alice pointed out when we were talking about this earlier, it looks like one of the basic problems in the previous setup was in differentiating between neutral and friendly characters. The forums and comments threads list out a few more, including where to stand for healing and what to avoid because it's damaging during raids.