Pearl fox

Pearl foxes are a grey color because the Pearl gene is a diluting gene, making the black pigments look grey and red pigments a soft orange shade. Pearl is a recessive type and does not show in heterozygous form. The shade of grey depends on the type of Pearl. Throughout history, multiple types have popped up in different countries. All but two are now thought to be extinct. The ones remaining are 'regular' Pearl and Mansfield Pearl. 

Mansfield Pearl has much more of a clear blue shade than regular Pearl. It does however come with its own issues: a high chance of Chediak-Higashi syndrome and lack of blood clotting factor. This is also the reason the Sapphire mutation is often sickly, one of the genes needed to make Sapphire is Mansfield Pearl. 

Regular Pearl foxes don't have these issues, which is why European fox farmers exclusively breed regular Pearl. Interestingly, regular Pearl foxes have thicker skin than other mutations.