Yellow could mean more than needing supplemental deco gas. But as was already said you'd need to have that in the dive plan and coordinated with the surface team/vessel. It could mean come and look for a slate or wet notes page indicating the nature of the problem. Missing diver, diver with a medical issue that may require evac, etc.. You'd let the surface know before you splashed that if they saw the yellow marker that something was wrong and to come check it for the aforementioned note or slate. Or to send a diver down to assess the situation and let topside know the details.
Hopefully whatever is going on can be dealt with by those on the surface in some way. Including getting help on the way if necessary.
My main signal devices are a DAN tube -orange, 50 lb lift bag-orange, 36 inch orange SMB, and 36 inch yellow SMB. The dive will determine when I carry the yellow one or the lift bag. Simple checkouts and recreational limits quarry dives? The 36 inch orange and the lift bag since any real emergency is going to involve a direct ascent without mandatory stops. Inland dives with deco - lift bag, and both 36 inch SMB's since it's highly unlikely I'll need the 6 footer to be seen over wave tops. Open ocean -DAN tube, 36 orange and yellow, and the lift bag. All of which is easily stored.