Hi Dani,
While I post infrequently and lurk constantly, I have been a carver of gunning decoys since the mid 1980's. I have never carved a Ruddy, but have access to paint schemes. "Decorative Decoy Designs/Diving Ducks, Volume II, by Bruce Burk copyright 1990 covers the painting of decorative decoys.
What follows is Mr. Burks' recommendations for a "Ruddy Duck Male/Winter Plumage."
Head & Neck
Upper bill - white,raw umber
Lower bill - black, white with some cadmium orange
Forehead and crown -burnt umber edged with white and raw umber
Cheeks and chin - white, raw umber
Neck and hind-neck - white, raw umber
Body
Back, scapulars and rump - Burnt umber speckled with white, raw umber & raw sienna
Sides - White, black, raw umber speckled with white & burnt umber
Chest - White, raw umber. Feathers edged with white, black & raw umber
Flanks -burnt umber, raw sienna, white. Feathers edged with white, raw umber raw sienna
Tail
Upper surface - burnt umber, raw sienna
Lower surface - raw umber,raw sienna, white
Wing/Primaries
Burnt umber, raw umber, white
Whew...no wonder I've never carved a Ruddy. Well, you have what you need if you're painting a decorative, and way more than you need if you're painting a gunner. I hope this helps.
Stay safe,
Dave