I have only one, it is yellow, and I put "DB" on it in black felt pen .. I thought I might as well let them know who's it was .. besides , it could mean Diver Below too 
I have talked to tech divers that have two and the yellow one is used to indicate trouble .. but was told that my yellow one is OK , I should mention it to the boat though .. as has been pointed out, yellow is not universal trouble .. .. and you should always mention what color(s) you have to the DM and and what it means to you anyway

I have talked to tech divers that have two and the yellow one is used to indicate trouble .. but was told that my yellow one is OK , I should mention it to the boat though .. as has been pointed out, yellow is not universal trouble .. .. and you should always mention what color(s) you have to the DM and and what it means to you anyway