Currently, the tranquilizer gun is pretty okay and it has its use cases, but generally the revolver is better. The colourblindness effect is interesting as a way to get players to not notice you approaching for a backstab/crit butterknife, while still giving a genuine chance for them to notice.
HOWEVER, in most instances getting tranq'd just results in the spy immediately being killed before they can get into melee range, whether it be by sentry or the tranquilized player turning around and seeing a spy stood with no other players around.
As a fix, I think the tranq gun should EITHER:
A: not break your disguise when fired and instead consume half your cloak meter (if you don't have half your cloak meter the disguise is broken when fired)
or
B: firing the gun doesn't break your disguise instantly, however 5 seconds after firing the disguise breaks so you need to get the backstab quickly and sneak away before other players notice