I think that you should also look at the way that non-printing characters are displayed within netbeans. At the moment
it only (as far as I know) allows you to color a while space block by using a color like which I think looks very ugly.
And seeing as I am a user of HAML a lot and tracking down bugs with how many spaces something is indented is an issue
that I have time to time, having non-printing characters look nice on screen is important to me.

I think you should allow the user to select the character and color of character that will be used for this. E.g. use a
dot, the whole character block, a tilde, a comma, whatever. But the user should have the choice over how the various
non-printing characters (White space, end of line, tabs) are displayed.