Why are my porcelain crowns light blueish/gray?

Likely PFMs: How old are they? Is the color mostly at the gums? Pfm's are porcelain fused to metal crowns so there is a layer of metal under the pocelain that can transmit light through the porcelain where it is thin. If this is the case then you could have them replaced with metal free crowns for esthetics.

Dark stump shade: The shade of a porcelain crown can be created by the ceramist who can stain the porcelain. Ceramist often will thin out the porcelain at the edge of your tooth appearing translucent and bluish to give a natural look. Gray colors are usually a result of underlying tooth structure being dark that may bleed through causing the porcelain to appear to be gray.