I still don't get the concept of having a favorite color. That's like having a favorite musical note or a favorite letter of the alphabet. It's not really anything on its own, it only becomes something meaningful when it's combined with something else (like a shape, or another color).
I can understand this. I've never had a favourite colour really. But I can imagine it being a thing. You're right that colour is not a thing on its own, but I could definitely visualise a mindset of the type:
"I am more likely to enjoy/appreciate [a given thing of shape X, or thing which is partly-coloured Y] if it also happens to be of colour C."
That's the sort of mindset that I can definitely imagine people getting into. And in that situation I think such a person would be totally justified in saying that C is their favourite colour.