The aircraft only get the yellow tails if they are painted. The billboard titles are only decals and are put onto the plane when they are in maintenence.
It would be nice to see an A321 with the yellow tail. I found a 757 in the new colours with yellow tail on avsim the other day (if you want me to show it I will do). Hopefully we will see all the aircraft types with the yellow tail but I dont think so as all the A300 & 757's will be gone by the end of 2014.