Lighter / darker pen tool?

Maybe I am overlooking this?
A pen that can darken or lighten pixels by a set amount.

I have used the free Piskel program, it has this feature and I think it is really useful.
In Pyxel Edit there is the color window with the shading and lightness option, I know. But a pen that can do this, alternating darker/lighter with Control or Option would be great!

Again, maybe I just don't see how to do this quickly :)