In Big Bento Modern, they are defined in system-colors.xml:
<color id="plugin.waveseeker.background" value="40,42,48" gammagroup="PlayerDisplay"/>
<color id="plugin.waveseeker.wave_normal" value="80,82,88" gammagroup="PlayerDisplay"/>
<color id="plugin.waveseeker.wave_playing" value="127,143,239" gammagroup="PlayerSliderLight"/>
In Winamp Classic Modern, they are defined in elements-color.xml:
<color id="plugin.waveseeker.background" value="31,31,54" gammagroup="plastic"/>
<color id="plugin.waveseeker.wave_normal" value="148,147,165" gammagroup="greybuttons"/>
<color id="plugin.waveseeker.wave_playing" value="138,196,64" gammagroup="display-text"/>
In modern skins where the above colours are not defined, the colour of remaining time is the same as wasabi.list.text. The colour of elapsed time varies. In some skins, it is the same as wasabi.list.text.current, in some other skins, I don't know what defines it.
In many classic skins, the colour of remaining time is identical to Normal in pledit.txt, and the colour of elapsed time is identical to Current in pledit.txt.
In WACUP's built in Default Classic, the colour of remaining time is roughly the same as the colour of normal text (#00F800 vs #00FF00) and I don't know what defines the colour of elapsed time. All I know is that it is a darker shade of green (#00B200).