A game of colors
The Void is a purgatory-like place you have wound up in as a lost soul. Things seem pretty grim but fairly soon you will come to know that escaping the place is possible. You can get help from the pretty Sisters who inhabit the place. Some of their uglier counterparts, the Brothers, also offer guidance but eventually all of them will turn against you. There are also few other creatures of different sizes and levels of threat.
Color is scarce in the Void and everything revolves around it. All of your stats are determined by how much you are holding it. You gather Color from various sources as Lympha, which in turn can then be placed in your Hearts where it acts as your health and provides different bonuses unique to each Color. Filling your Hearts with Emerald, for instance, decreases damage you take from attacks.
While traveling the Void, the Color in your Hearts drains and turns into Nerva. As Nerva, Color can be given back away. This happens most efficiently by using Glyphs which are essentially spells you cast by drawing their shape on the screen with your mouse. The more Color is used (the longer you draw the shape), the more powerful is the Glyph.
Too stressful to enjoy
While I greatly enjoyed the aspects of the different colors, The Void was not a very pleasant experience. It is a very difficult game to get into due to how easy it is to screw things up irredeemably. I suppose the game does explain you its mechanics but truly knowing what exactly you should be doing -- essentially getting good at it -- takes a while.
I restarted from the beginning and had to reload earlier saves multiple times. Like when a Brother suddenly engages you into a boss fight and you happen to be out of Nerva'd Color due to just having challenged another Brother yourself, you are simply shit out of luck. I eventually had to look up a guide for better strategies to get through the game in a tolerable time frame. The Void's working-title for its English release, Tension, suits it well.
Needed a better interface
Another thing that I disliked was how much you need to micromanage. Every time before entering the Void, i.e. the map screen (where time passes), you have to empty your Hearts from Color you do not want to get drained into Nerva. And then after entering a chamber (where time stands still), you again have to refill your Hearts for desired bonuses, such as Silver to not damage a Sister's realm too much when using Color to liven things up.
The color menu interface is like designed for touch devices but I do not recall them being very common in 2008 when the game was originally released. It would have been easy to assign a hotkey for each Color since there are only seven of them. I would have probably ditched the whole drawing thing too, although assigning hotkeys for the 21 Glyphs would be more difficult. Maybe some sort of cycling and/or power wheel solution could have worked.
The game's ending was a letdown. It did not give me the feeling of satisfaction. There are multiple ones too -- one ending for each of the 11 Sisters. I watched them on YouTube and every one of them was similarly disappointing. You get a different poem recited (no subtitles) over a cutscene that differs between the endings mostly in color.
The Void uses Ice-Pick Lodge's own engine, though it reminded me of id Tech 3 used in American McGee's Alice. Environments are similarly barren and the minor enemies are kind of low definition. Color glows are beautiful, however. And the Brothers and Sisters are quite well detailed too. The Sisters in particular may be of eye-candy because in dialogue they are only covered in color particles. Particles which get less covering as you fill the Sister's hearts. Not that the Sisters are normally all that clothed.
I also found an intriguing theory about the game's message when browsing for opinions about it. I am personally quite hesitant about interpreting things that are not clearly stated in a piece of art, especially when it comes to video games. It is so easy to take into consideration only the evidence that supports your theory and ignore the rest -- to only see what you want to see. I remember one awkward movie review on a local newspaper about The Lord of the Rings and its supposedly obvious sexual themes . . . Regardless, I found the aforementioned essay an interesting read.