Late to the painting party here, but here’s my take on ‘pop’:
Really, it’s all about ‘read’ at small scales – can the eye differentiate between the different sections and parts of a model.
Bright colours help to do this easily, so that’s what it’s about is my feeling. Which is why I like to use that red/terracotta colour instead of true earth tones.
I’d have gone turquoise on eel and btw, but then I do use too much turquoise!