Were DB Branch Line Cars red?

If we look into the 2002/2003 Märklin catalogue and open page 161, we will find the “4335 Passenger Car” and the description reads: “German Federal Railroad (DB) type Bie standard design branch line car.” And we see a red passenger coach.

So: was red the standard colour for branch line cars? No.

If we go back to the 1994/1995 Märklin catalogue and open page 100, we will find the green “4235 Passenger Car” with the description “Bie standard design branchline car” and the red “4335 Passenger Car” with the description “Standard design Bie branchline coach”.
So what? Well, “standard design” refers to the shape only, not to the colour. Green was the standard colour for DB branch line trains in era III.

But as the catalogue also explains: some branch line cars had been painted red for use with red powered rail cars (and also received a modified heating). After the powered rail cars had been retired, the red coaches were used for locomotive operated branch line trains.

And some push/pull branch line trains with locomotives also used red coaches.
So yes, there were red coaches used with passenger trains, but they were rather rare and dark green was the standard colour for passenger coaches in era III branch line trains.