In all plumages, Ferruginous Duck is essentially dark brown with white undertail coverts.
Adult males are distinctively white eyed, whereas females and juveniles have brown eyes.
Ferruginous Ducks can easily be confused with female and juvenile Tufted Ducks.
However, Tufted Ducks typically show at least a hint of a tuft or more often than not an obvious short tuft on the hindcrown.
In addition, the head shape of the Ferruginous Duck is rather peaked and the female has dark eyes whereas female Tufted Duck has distinct yellow eyes.

1st & 2nd photo: A male. Kromslootpark, Almere, The Netherlands, 11 January 2022
3rd & 4th photo: A female in the company of a male Tufted Duck. Zanderij Crailoo, Hilversum, The Netherlands, 12 September 2018
5th photo: A female. Genemuiden, The Netherlands, 13 February 2010