Can't change background colour of linear bar vertical slider

Hi, I’m using linear vertical bar sliders like it describes in this post here:

and now I’m simply trying to change the background colour however I can’t seem to get it to work.
I have tried:

setColour(juce::Slider::textBoxBackgroundColourId, juce::Colours::grey);

and

setColour(juce::Slider::backgroundColourId, juce::Colours::grey);

but neither of these works.
How am I supposed to do this?

Thanks