It all started when I noticed that my eyes were quickly growing tired when coding. I had been using default Ubuntu colors back then (white on dark magenta), and the contrast was simply too high.
Selenized has moderately low contrast - a bit more than half the distance between pure black and white. The result is easy on the eyes, but still very readable: long coding sessions are not a strain anymore! (compare with Ubuntu colors here)
Lightness of accent colors needs to be carefully adjusted, so that they are both pleasant and present an even contrast against the background. This is tricky because of things like Helmholtz–Kohlrausch effect and eye cone sensitivity differences.
Selenized harmonizes the lightnesses while preserving each color's individuality (e.g. yellow should be brighter than red). This is possible thanks to the use of perceptually uniform CIE Lab color space.
Many palettes - including default coloring in xterm, Ubuntu terminal and Sublime Text - weren't designed this way: they have a lot of variation in lightness. You can see above how this leads to bad readability.
Selenized includes four variants so that everyone will find something that suits their taste. Thanks to the magic of CIE Lab color space, all variants share the same lightness relationships, resulting in exactly the same readability.
Like Solarized, but better. Dark teal and warm sepia complement each other nicely.
Contrast inside terminal/editor is one thing, but what about the whole desktop? A window with black text on white background (e.g. a pdf document) next to your code will influence its perceived brightness. Also, what if you are working outdoors?
That's why selenized has black and white variants: oldschool look with a little more contrast. Notice that yellow color on white background is readable here.