These letters are done with SVG fills. Unless the other alphabet project that uses SVG strokes, each of these letters (or parts of letters) has an outline around both sides. Each is a distinct shape that has a fill color but not stroke. The shapes are rectangles, polygons, circles, ellipses, and paths. The paths include bezier curves, quadratic curves, and lines (but no arcs). Some of the closed letters like D and O use masks, and some letters that use partial circles, such as C and G, use clipping paths.
Each letter is in an individual SVG file. The colors are assigned in a special JavaScript file that is included in each SVG file. The colors are assigned by adding them to the SVG file's URL, for example:
a.svg?purple,violet