Not rarer, but I do think different colours denote different ores to mine. And they don't actually change colours, the node vanishes and is replaced with another. So you should always mine every node you see immediately.
edit: at least that's how I play it. Someone correct me if I'm wrong.
I do believe they are different based on rarity or maybe the better word would be quality. Like in the volcano, you'll get better results from the bluish colored ones vs red ones. But the places they appear are fixed.