The problem is, that the colour is used in a sub component of the slider, a Label named valueBox.
When you set the colour in the lookAndFeel, no update is triggered, but if you set it to the slider, it will trigger a colourChanged(), which triggers a lookAndFeelChanged().
If you call FreqSlider.lookAndFeelChanged() in your initial version, you should see the changes taking effect.
Btw, don’t set the colours in paint, just do it once in the constructor. Worst case such a change could trigger a repaint() leaving you in an infinite recursion…