Depends. There’s important context missing. Was the 5 in my hand when any of the colour clues that touched my rightmost card were given? (So that requires three new draws into the hand since the last colour clue, which is a lot with only eight cards played. This game is going badly.)
If the 5 was in hand when any colour clue was given, then it is not 5m, I have to believe that C cannot see 5m, and thus if I play my multi, C will play their multi as 5m. So it must be trash bluff, and I have to play my blind card to tell C that.
On the other hand, if I don’t have not-multi on the 5, things get more complicated. In that case, either C can see 5m in my hand, in which case the clue marks known trash; or C cannot see 5m in my hand, in which case I have to respond as trash bluff. Really, I’m forced to guess.
I don’t like guessing. I would look for additional context. What is C’s chop, and do we need to preserve it by telling C to discard the trash immediately? Do I have a token, and can we afford to use it? Does C hold another 2? Any 5? What colour was C’s card marked before the yellow clue?
The safest approach would be to clue 2 or 5, as long as they don’t touch any other card. Either of these would tell C clearly to discard the multi. (5 works because my card should be 4m by Good Touch, so any non-5 multi should be trash.)
If I can’t clue 2 or 5, I think I would probably play my blind card.
But honestly, if the position isn’t desperate, this isn’t a clue that A should give in the first place. There will almost certainly be better opportunities to clean up that trash.