I'm assuming here that the visualiser that is now available in the Playing Track tab has the same two values as the "SpectrumVisualiser" element in Theater Mode. A "fg" and a "bdr" so:
<element id="NowPlayingPanel.SpectrumVisualiser" fg="30,255,255,255" bdr="30,83,91,18" />
just like you made "SoundGraph" into
<element id="Player.Wavebar" bg="69,114,59" fg="53,109,48" fg2="201,206,150" />
If don't want to do a new skinning element, then maybe it could use the colours from
<element id="Player.SpectrumLow" bg="" /> and <element id="Player.SpectrumHigh" bg="" />
instead.
It sort of defeats the purpose of having the visualiser there, if the greys and off whites it currently uses, blend into the artist photo behind it.