The second one contains spoilers for the game, so proceed with caution!
In the comic, the background is black, but for the wallpaper, I wanted to do something similar to the colorful special effects we see in the background of the battle with Asriel. But instead of glitchy abstract patterns, I wanted this background to reflect Sans' psyche, and to use some of the colors associated with his soul.
The pale blue is one of the colors his eye flashes. In Undertale, this color is associated with patience. The twelve stripes represent the twelve hours on the clock, which in turn represents time and the time loops that have become the bane of Sans' existence.
The dark blue is the color skeletons turn your soul when they use blue magic on you. It's vaguely heart-shaped in this picture, but fuzzy and tumultuous, like the distorted personalities of the Lost Souls.
At the edges of the image, the yellow associated with justice - the other color Sans' eye flashes - and the purple of Karmic Retribution lurk. These aspects of Sans don't come into play in the pacifist route, but they're always there, just in case you decide to betray him.
All four of these colors are a bit darker in this image than they are in the game; at their full brightness, they made the background far too loud and busy, and threatened to drown out Sans, so I toned them down a bit.
You can see a larger version of the wallpaper on my DeviantArt.
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