[PATCH 3 of 3 V2] color: change color of grep.rev label

[Jordi Gutiérrez Hermoso]

On Wed, 2019-02-20 at 11:49 +0900, Yuya Nishihara wrote:

> I've marked this as "(BC)" since this is IMHO a significant change.
> Blue on black is unreadable on my machine.

It's kind of hard to predict what will be readable. Linux terminal
emulators usually let you define "blue" to be whatever you want.

I wasn't sure if colour changes were BC, but I did put them in
separate commits so they could easily be separately dropped. I'm okay
with not changing the default colours and just adding more stuff to my
config. The only part I really wanted was extra labels.