If there’s a requirement for an accent colour that isn’t orange or purple, I think the aquamarine you suggested in the other thread is worth exploring. If I try it on the volume slider, I think it looks better than the current shade of blue - in that one context at least:
Since it’s directly opposed to Ubuntu orange on the colour wheel, then it might make it a sensible addition to the Ubuntu palette.
However, in the volume slider mockup, there isn’t any of the Ubuntu “light aubergine” in shot, so I guess you would have to check it looked appealing in all Ubuntu contexts, rather than just the volume controls?
EDIT: full screen mockup for more balanced view.