Waveform color scope: Use system palette for better theme integration + fix bug in green paint mode

This includes also a bug fix for the Green paint mode. It produced brownish and other weird colors instead of green as 0 values were not handled correctly which produced -inf when calculating its log value.

Green Paint mode fix: Before/After

As With the RGB Parade PR I did not change the dark2 custom background for the Waveform background as the paint mode were built with a dark background in mind.

before

Themes: Breeze light & Sweet

after

Themes: Breeze light & Sweet