Your comments

I see that the color of the invisibles is somewhat derived from the color of the current scope. 

Then probably a setting to control the alpha/opacity of the invisibles, so that one can make them a bit more/less subtle if desired.

+1 for this one (sorry, at first I didn't see the votes box)

This issue is related to the draw_white_space setting (esp. when set to "all"). Whitespace chars are currently drawn using too heavy colors.

Other people want it too: