• This is quite close to informal.
• It is mostly used with speech rather than writing, though not necessarily so.
• The word is also slightly stronger on average than informal (i.e. more informal).
• It suggests a yes–no qualification: saying more colloquial is not so common.
• It is usually neither positive nor negative, nor felt to be lower class.
• However, the euphemism "colloquial at best" is often used to mean that it is bad style, referring to a colloquialism used in the wrong setting.