Pixel MapA pixel map is a style of map done on an aliased raster image - that is, it uses a limited set of colors and does not do any blurring between them, making it quick and easy to edit compared to anti-aliased images. Each pixel is deliberate in its placement and meaning. Take a look at the map above: the black color that separates the blue from the green is the coastline, and it is one pixel thick, which is the bare minimum needed to convey the separation and is the most common style of coastline marking.
Pixel maps are widespread across mapping communities such as alternatehistory.com. To begin creating them, a simple raster image editor such as Microsoft Paint will work, but there are more powerful alternatives such as Paint.NET and GIMP which offer features such as layers and proper .PNG image support. The former has a low learning curve and is an excellent choice for beginners, while the latter offers a few features that can be very helpful for more complicated map editing.
Color PaletteThere are many different color schemes across the mapmaking community, and there have been several attempts to unify these standards[x]. But what these standards all have in common is their colors for land, coastline and the sea. Depending on the type of pixel map being made, one of two color schemes will generally be used.
Political Pixel MapsPolitical pixel maps are colored based off of a color code where each color relates to a specific country. There also exist various colors for things such as subdivisions, autonomous regions, rivers and more. The image below shows the ARCS color scheme and a worlda map1 - the worlda map is a very popular basemap for people interested in creating alternate history maps. In general, most colors align properly between the most important countries (e.g. Japan, Germany, Canada) in the different color schemes.
Alternatehistory.com has some great resources on color palettes and how to draw pixel maps. Be sure to check out their forum board which has thousands of pages of examples for anyone who wants to get a feel for the style of pixel maps.