Or if you meant "when did the last predecessor unit still in predecessor paint operate with Conrail?" the answer is probably about 1985.
I saw a RDG GP30 in 1983, several PC black units in 1984...but nothing but solid Blue from 1985 and after.
a scan through RP.net shows:
some PC black still around in 1984.
a PC black GP9 in 1985, but could be stored.
(stored units still in predecessor paint are still out there today!)
but no units obviously operating in 1985.
RDG GP30 in 1986!
also looks stored (notice the numberboards are gone)
PC black in 1986, but doesnt count, its a stored unit:
EL in 1986, but appears to be stored:
and I think that it..going into 1987 and '88 the colors are gone.
nothing but Blue.
my personal assessment is:
1984 - still some noticable predecessor paint here and there.
1985 - almost gone..hard to find.
1986 - its all over.
1985 was probably the last year.
I never saw another one after '84.. and I was trackside along the Southern Tier Main nearly every day.