Yellow hydrangeas don't exist irl, but the flower itself can represent honesty and gratitude as well as a deepening friendship. The color yellow probably reflects acknowledgement of her title/status.
I immediately find yellow hydrangeas with a simple Google search. Apparently they are a Hardy plant. Yellow symbolise joy, happiness and friendship in flower language
uhhh...her husband is just beside her??? HAHAHHA i mean its not wrong since you just want to congratulate but the flowers and in front of our easily jelly daddy claude plus he looks like a potential source of jealousy HAHAHAHAHHHAHHAHA the guts dude