Also, as far as the gray--because it's so light, it'll work better with a white shirt for days when you want to look particularly formal. It isn't dark enough to work with blue (not enough contrast)
Exactly. Since ties should be generally darker than shirts, ties that are in the light blue or light grey range are going to look better with a white shirt. Same idea with a light blue tie and a grey suit (obviously not with a light blue shirt but say a pink shirt - navy or at least a medium blue would be much better).