Am I full-blooded white?

I have blue eyes, brown hair, and no non-white heritage at all. But my grandmother was born in Mexico an came from a wealthy, all-white, pure Spaniard family. Her grandfather had raised her parents (who then raised my grandmother) to keep their white heritage pure, and to never mix with another race (they aren't racists, but they have Spanish royalty in their blood and want to remain full-blooded whites). She has brown hair (now gray), brown eyes, and is just as white (and Spanish, heritage-wise) as Hernan Cortez. So does that make me a quarter Mexican, or am I still all white?