'Dayglow' colors would be very bright colors that can be seen easily even in bright daylight. Safety colors worn by construction workers, like blaze orange and safety lime, are examples of 'dayglow' colors. The girlfriend is referring to a style of very bright, inexpensive wall-art that is sometimes associated with teenagers and college students. A Worhol, by contrast, would be a very expensive and sophisticated piece of wall-art.

Thank you for the extra explanation you provided, Anna!

Velvet paintings were a style of folk art in the 1970's (?) in the U.S. They are considered rather tacky (unstylish or uncool). They were often done in "dayglow" colors so that they would shine under a blacklight. The cliché or stereotypical velvet painting would be a painting of Elvis Presley ("velvet Elvis"). A synonym for dayglow would be neon. Search Google Images for dayglow velvet painting to see some examples.