There are many reasons why a piece of media is forgotten to the sands of time. Sometimes it's because something isn't very good, sometimes it's because it was released at a time that prevented it from getting a better foothold. In Colorful's case, it's because it just wasn't very memorable.
So what's the plot for this series? It doesn't have one! Based on a manga series, Colorful is a series that focuses on the misadventures of men staring, peeking, glancing, looking, and glaring at women and their efforts to get that extra eyeful of harmlessly exposed panties, bras, and occasional cleavage and the outrageous consequences of their actions.
It's more like a sketch show with a few re-occurring characters and exactly one sketch rather than an ongoing series, which should be pretty obvious with it's tagline "Roses are red. Violets are blue. This show's about panties."
Some series have a character that's a pervert for comedic relief, or even have it as the main focus, but Colorful distills the perverted comical relief down into 96 minutes, consisting of 16 short 6 minute episodes. The whole thing pretty much feels like a montage of the slapstick pervert characters from a bunch of other longer running series. By the end of the series it definitely feels a little long in the tooth, and it doesn't have the benefit of being broken up by an actual story or other events going on. It's ecchi comedy distilled into the length of a feature film.
I'd complain that a series with this little substance got made, but there have been series out there with somehow even less substance. And at least Colorful doesn't pretend that it's something that it's not. If anything, I can give this series credit for at least being interesting with it's animation and art style, often being mildly creative with the way it looks.
Coloful has been described as a very love it or hate it show, seemingly being described as "wildly funny" or "tasteless and depraved", and it's easy to see why. It's very unashamed of what it is, a series of vulgar panty jokes one after another with very little breathing room. I'm pretty sure there is an audience out there for something like this, but it would be an incredibly niche one at that.