Learn French: J'affirme ceci : ce compromis est inacceptable. - I state this: this compromise is unacceptable.

In formal French, ceci is specifically used to introduce an idea that is about to follow, like the statement after the colon. If you were referring back to a statement you just finished making, you would use cela.

Both ceci and ce translate to "this". How are their roles different in this sentence?

Ceci is a demonstrative pronoun, meaning it stands completely on its own to represent "this statement" or "this idea". Ce is a demonstrative adjective, meaning it cannot stand alone and must be placed directly in front of a noun, as it is here with compromis.

Is there a specific reason a colon is used right after ceci?

Yes, the colon acts as a bridge. Because ceci announces that a specific explanation or quote is coming, the colon perfectly sets up the reveal of what "this" actually is. You will very often see ceci followed by a colon.

The word compromis ends in an 's'. Is it plural here?

No, it is singular here, which we can tell because it uses the singular adjective ce and the singular verb est. The noun compromis simply has a silent 's' built into its singular spelling. To make it plural, the noun itself does not change, but you would update the surrounding words: ces compromis sont.