From the Muppet Wikia,

The song was described in a Children’s Television Workshop press release as “a frog’s poignant realization of his own dignity and worth.”

Since being penned by Joe Raposo in 1970, the song has been universally embraced as its theme can lend itself to so many issues including race, gender, and self-esteem amongst others. And while it has been recorded several times by the likes of Jim Henson, Frank Sinatra and Steve Whitmire – The song really grew into its own when performed by legendary musician, Ray Charles. Performing the song in 1989 on “Sesame Street: 20 and Still Counting”, Ray Charles, and a chorus of Muppet singers made the performance one of the most notable in the history of the show.

Lyrics:
It’s not that easy bein’ green;
Having to spend each day the color of the leaves.
When I think it could be nicer being red, or yellow or gold-
or something much more colorful like that.

It’s not easy bein’ green.
It seems you blend in with so many other ordinary things.
And people tend to pass you over ’cause you’re not standing out like flashy sparkles in the water- or stars in the sky.

But green’s the color of Spring.
And green can be cool and friendly-like.
And green can be big like an ocean, or important like a mountain, or tall like a tree.

When green is all there is to be
It could make you wonder why, but why wonder why? Wonder,
I am green and it’ll do fine, it’s beautiful!
And I think it’s what I want to be.