I arrived at the head, input my Red and Blue jewel. Turned out my Green candidates were all wrong, so I went back and got what I think is the right one.

Once I got back to the head, the Green crystal was now sitting in it instead of the Red I put in earlier, so it broke my game since I had used up my Red one to put in the head in the first place.

I had to reload a save fairly far back so to save yourself some headache make sure to input the green one first so the game can't mix them up for you.