An expression is said to be an example of metaphor if a key word or certain key words have meaning which transcend the literal or dictionary denote.The word "fin",though it can sometimes be slangily used to mean the human arm,has meaning far beyond what the dictionary can describe.
It depends. Which bits are literal? What is 'he'? What's the context?
If 'he' is a fish with literal deep purple fins, then we're left with explaining "shone with dignity".
Otherwise, "deep purple fins" could be metaphoric.