What does static gray mean, in following paragraph?
Over time the crossings allowed the genes for white fur from the Angora to become a stable part of the Persian line, changing the coloring of the Persian from a static gray.
Adjective 1. lacking in movement, action, or change, especially in an undesirable or uninteresting way.
'Static grey' here means the fur was always this color. It never changed or was different.
So, it means that with the introduction of the new genes the Persian's fur transitioned from a consistent, non-changing grey color to new colors.
Actually, I think that static-grey here could be a description of the exact grey colour. Static Grey likely refers to the overall grey tone you see when a TV is not tuned correctly. If you google Persian cats they do vary in tone but all are grey - static grey suggests the most common colour is a light-grey colour as opposed to a dark grey.
Grey and gray are interchangeable :)