my girls had yellow/orange bills and feet until they were a year old... then they gradually got darker. one of my ladies who is three still has orange/black mottled feet. But she's the one to go broody the fastest.I can only find Australia's and England's SOP. Both say "dark slate" for feet and bills.
I also looked back at pics of my duck, and her bill didn't get really dark until she got a little older.
How old are your ducklings? Here's Marmalade at 2 weeks. You can see her bill and feet are still greenish/orangeish
View attachment 3307026
And here she is at 4 weeks. Much darker bill and feet. Her fet are nearly black now at 1.5 years.
View attachment 3307031
Maybe yours will get darker?