The Four Color Theorem states that for any separation of a plane into contiguous regions, producing a figure called a map, a maximum of four colors are required to color the regions of the map so that no two adjacent regions have the same color. This fact might seem of non-trivial interest only to geometricians and cartographers, but, fortunately for us, developer OneFifth noticed another interesting property: it makes for quite a nice basis for a simple idea puzzler! That game is Flood Fill, and there's no doubt it deserves a place on your browser gaming map.
In each level, click the mouse to fill in areas of the map with one of four colors: blue, green, orange, and pink. Switch between colors by clicking them, or using  or [ASDF] as keyboard shortcuts. Each level is finished when all areas are filled in, with no two adjacent (which is to say, meeting at a side, rather than a corner) regions sharing the same color. All levels are proven a 1976 brute force proof to be doable in four colors, but a gold star can be earned by using fewer. Flood Fill isn't particularly complex, but it's surprisingly engaging. It's a premise that could probably be sustained for longer than the 20 included levels. We could probably quibble about the aesthetic value of neon, but hey, the background music is catchy. So few games offer you the chance to make like Piet Mondrian in the comfort of your desktop, and Flood Fill will fill a coffee break quite nicely.