The enumeration for slider colours doesn’t have anything for the inc/dec buttons, and none of the other ones affects them. The incButton and decButton members of Slider are private, so they can’t be accessed directly.
Is there a way to change the colour of the inc/dec buttons?
For now, I have declared incButton & decButton public: in juce_Slider.h and called setColour on the buttons directly, but that has a pretty bad smell to it.