Some people see days of the week as colors. The week becomes a stripe across the inner eye. Pick yours.
Grapheme-color synesthesia is the phenomenon where letters, numbers, or units of time consistently appear in color to the perceiver. Day-of-week color association is a related variant. It's more common in childhood and runs in families. The colors are usually stable for a given person across years — your Tuesday is your Tuesday, even if mine is a completely different blue.
The picker uses HSL because the slider direction is where the meaningful variation lives — hue around the rainbow, saturation from gray to vivid, lightness from black through the hue to white. Native 2D color pickers waste most of their area on near-identical darks. Submissions are anonymous; an opaque client ID stored locally lets you refine your entry without spamming the aggregate.