You're thinking your appearance needs some spicing up, and that you wouldn't mind adding a little color to a secret place. But you also know that, if you're going to invest your money in a permanent bit of body décor, you need to put some thought into its design.
So you head for your friendly neighborhood tattoo parlor (with the skyrocketing popularity of tattoos you're bound to have one) to look for inspiration. You know that whatever you select will become a permanent part of your existence. You can cover it up, but you can't escape it. So you will need to choose carefully.
The first thing you may notice when you enter your tattoo parlor is just how many designs there are. It's not unusual for every nearly square inch of a parlor's walls to be covered with posters, nearly every square inch of which is covered with designs. You are experiencing "flash", and not because all of those designs and colors make your eyes feel like that a flashbulb just nailed them.
Tattooing was a staple of traveling carnivals in the early to mid 20th century, and carnival barkers were notorious for the shelves and shelves of cheesy gaudy prizes which they used to lure players to games completely rigged to relieve them of their money. Those prizes were known, among the barkers, as "flash", and it may be that the carnival tattoo artists adopted the term for their selection of tattoo designs.