In settings I’ve enabled the “Custom color for completions” checkbox, but I can’t work out how to set the actual color. Any pointers gratefully received 
In the end the Copilot settings pointed me in the right direction.
Settings… Editor > Color Scheme > Language Defaults … Inline Hints > Text without background
This changes the color regardless of whether you check “Custom color for completions”, so I’m still not sure how that setting is supposed to work.