Chris Colfer says he was warned not to come out as gay while on "Glee" -- even though he played an openly gay character -- because it'd ruin his career ... but he didn't listen.

The actor went on "The View" Tuesday where he reflected on his time on the hit show from the 2010s -- on which he played Kurt, who was gay ... obviously mirroring CC's real-life sexual identity.

At the time, however, Colfer says he wasn't out just yet ... and goes on to say people around were encouraging him to keep his orientation a secret -- warning he could torpedo his career.

According to him ... the thinking from these folks at the time was something along the lines of -- don't come out, it'll only hurt you, and you have to stay closeted, it'll pay off professionally down the line. As backwards as this seems ... he says that was kinda the mindset then.

Interestingly, Colfer says he actually didn't come out for a while at first -- but when he got a sweet letter from a fan thanking him for the representation his character was offering on TV ... he realized that being a beacon of inspiration for others was more important.

Now, he lives out loud ... and has been with his partner Will Sherrod for a decade.

Of course, it's Pride Month right now -- so this story couldn't have come at a better time. Things sure have changed since when "Glee" was on the air ... that much is certain.