This is glitch on Mono Trakcs only (on the Sends slots).
Mono buses that are currently in use don't show up in "Yellow" to indicate it's being used.
For example: If bus 1-2 is being used on stereo track somewhere, bus 1 (Mono) or bus 2 (Mono) won't be in Yellow for when your setting a bus on a Mono Tracks Sends. So it looks like the bus is free but's really it's being used.
I'm currently using PT 12.5.0, 12 processor 3.46Ghz Mac Mid 2010 OSX latest El Capitan.