Mark has actually never outright told the rest of the members that he has synesthesia. He's nervous, okay!? Whenever he opens his mouth the words just don't come out and soon another member is filling the silence with chatter. Sometimes Mark really hates being the quiet one. 

(or, alternatively, the mark has synesthesia au no one asked for but i wrote anyway lol)

[actually, that's a lie, i asked for it lol]