1964 Omega Seamaster Help!!

My parents collected art back in 60’s and 70’s- they had started earning real money and had taste- and the art market was wide open back then. My father bought a lovely Marc Chagal signed, hand colored print during that period and it hung in our house for years- it has been appraised 3 times for insurance and has always been verified as original.
Flash forward, my parents are divorced, my mother gets the Chagal (they amicably split the art collection), and my stepfather (who hated my father) had an appraiser come in to assess their art collection for insurance- along with his questionable taste collection. The appraiser raises an eyebrow to the Chagal saying he had never seen “those specific colors” in a Chagal and questions it’s authenticity (this tool obviously didn’t account for the fact that it had hung in a Southern California home for 20 years- undoubted the colors has shifted from UV exposure).
My stepfather screams fake- it’s garbage, get it out of the house! It has no place in their collection. My mother, feeling rather crestfallen, offers me the Chagal- of course I grabbed it without hesitation.
It has been reappraised, it is real- and it’s mine.