And more on moult strategies of HOUSE FINCHES - Alex Lees

from BNA:

Prebasic I Molt. House Finches undergo one moult annually from July to October (Michener and Michener 1940, Stangel 1985). Prebasic moult complete except in Prebasic I moult, when a variable number of juvenile primaries and secondaries retained. In California, a few birds moult all primaries and secondaries and most moult at least some primaries (Michener and Michener 1940); in Massachusetts, no juveniles moult all flight feathers and 80% fail to moult any primaries (Stangel 1985). Duration of moult related to number of primaries replaced, so average moult duration for juveniles shorter in Massachusetts than California (Stangel 1985). Massachusetts House Finches begin Prebasic I moult with primaries 4, 5, 6, or 7 (Stangel 1985); California finches with primaries 3 or 4. Degree to which difference in Prebasic I moult between eastern and western populations reflects genetic differentiation or environmental influences warrants further study.extract from Hill (1993).

If we assume the Cornish male was unlikely to have 'vagrated' out to sea and onto a ship until October then there is a rather small time window available to complete the moult in the absence of dietary carotenoids. Moreover, it is unlikely to move during a moult sequence. More likely it was yellow before it (hypothetically) left the US (Alex Lees)