They are not extremely rare, but definitely not as common as the typical blue and green variants. Here's my pride and joy grey Princess Sheryl
HOUSE JEDI OF BUDGITON
"Let birds of ALL feathers flock together"
Thy Royal Court
Prince Kobe, Lady Gisele, King Chewie, Queen Betty, Princess Silverhawke (RIP 10/31/2015), Lord Artoo
Guardians of Budgiham Palace
Viserion, Rocco, Drogon, Rhaegal