Hi Julian,
Thanks for sliding in - hopefully this solution scales up to your expectations!
According to the comments in the PsychoJS lib it looks like HSV is on the To Do list but it isn’t in place yet.
What you could do though is convert the desired hsv values to rgb values to give the desired color in hsv. Sadly, psychopys hsv2rgb doesn’t yet exist in PsychoJs, but thankfully this question prompted a demo and think we might have a function to use now! Here is the demo of that with the code here (please note I am not a JavaScript programmer - but I cross checked the output of this function with the hsv2rgb function in psychopy and the output seemed to correspond!).
Hopefully this allows you to create what you need!
Becca