[plt-scheme] test-coverage and colorblindness

The syntactic test suite coverage feature prints tested code in green
and untested code in red. As far as I can tell, these colors are not
configurable. This color choice is very unfortunate for colorblind
users.

Would it be possible to make these colors configurable, and/or to change
the defaults to something better? I seem to recall that the tested-code
color used to be black.

Thanks!
~ Aleks