Day-glo, or Blacklight ink, is "ink that glows under a black light, a source of light whose wavelengths are primarily in the ultraviolet". The paint may or may not be colorful under ordinary light. It is also known as fluorescent paint.
So, I guess day-glo colors are colors that have a glowing shade resembling the Blacklight-reactive ink.
A vivid image to illustrate what I'm talking about:
Kate Middleton in an outfit of day-glo colors
So, I've been feeling excited about these colors lately.
These colors ARE exciting!
The combination of day-glo pink, green, and grey is the best.
One extra note: Bra straps in day glo colors are super hot!!!