Blue Light

Santa Claus is the only source, and he only lives in town during the Christmas season, so stocking up is a once a year job. Each color is a separate line on his list rather than something you recolor after the fact, so buy all three in one trip if you want to mix strands.

The glow is faint enough that it will not light a corridor for you, and treating it as real lighting will leave a room dim. It does still register for housing, so a strand run along a ceiling can legally furnish an NPC room while a lamp elsewhere does the visible work. They attach to all four faces of a block, which is what lets a run of them turn corners and read as strung wire instead of a fixture. They also answer to wiring, so a whole display can be switched off from one lever when the season ends.