I would like to be able to highlight a @window switchbar/treebar button with whatever color I specify, with support for double digit. I was thinking -KN (N = 0-15)

This would highlight the window with green.
/window -K3 @test
or
/window -K03 @test

$window(@test).sbcolor could return the word custom or the color name.

~ Edit ~
-K with no N could remove (reset?) the highlight.