Mine (vim) does that too, but 16 years or so of being trained that '!' is very, very special can't be changed by some color. I think @assert(), $assert(), or macro.assert() would be less frustrating. When asserts trip, the 'impossible' has happened so you don't need any extra cognitive load when chasing them down. Also in python2 assert is a keyword so assert!(x) means something very, very different.