On Win21 bbls,,Express Rust will someties result in the color fading a bit or turning reddish especially at the breech. The lower corners seem to be a prime candidate for the problem for some reason.
Express Rust I have used mostly Mark Lee formula for the last 30+ yrs.
The over all color can sometimes show a little reddish hint to it in brite sunshine as well, though not always.

I can't ever think of times when this occured when I still used the old Belgian Blue (from Herters at the time). That Express Rust bluing soln had the mercury bichloride compound in it and it surely worked well.
But I decided after some years of using it and others with the merc in it to drop those.

Perhaps the Mark Lee, which does not have any merc compound in it is at fault for lack of a better word for not agressively rusting the 21 bbl steel as well as the older solns.
Just a guess of course.

(An aside,,the return of the Belgian Blue that Brownells put back on the market some time ago I believe still uses the mercury compound. When Brownells placed it on the market, I did buy a small bottle thinking
maybe it worked as well as the old formula but had been re-formulated to leave the mercury out of it.
A simple experiment of swiping some of the soln onto a piece of polished brass that had been warmed up a bit lead me to believe the Mercury was still in the 'new' Belgian Blue.
When I wiped the soln over the warm brass, the surface was plated with silvery Mercury(?) justas the orig stuff would do to brass, gold, silver' copper , etc inlays or elements on the gun pars.
..The ads said the new stuff was same as the Old Belgian Blue,,guess it is!
I was just surprised that in the Age of Green and the extreme hype of any Mercury component in anything OTC that it would remain so. )

Back to Win21 rust bluing.

With the Express Blue process,, a slightly coarser finish on the steel,,coarser than I like for a Win21, seems to help the Express Rust process (Mark Lee at least) avoid the later discoloring or lack of color in spots.
It's no secret that a coarser finish is helpful to a rust blue anyway.

I've found that if I polish the bbls to any gloss I care to and then Slow Rust Blue them, the color problem do not appear.
For this I use Laurel Mtn slow rust bbl blue soln.
I don't let very heavy coatings of rust to form before boiling the bbls. Barely rusting so that you have to look closely to see the coating on the metal is enough to give you another layer of color. More rust is just that and muddys up the tank water. Plus the longer it sits and rusts, the more matted the steel can become.

I did have some problems with after rust using L/M rust soln in the past. Tried about everything to kill it. Some worked better than others but none really worked totally well.
I've settled on 2 now.
One is to right after the final carding of the last coat of blue using the L/Mtn soln, I then warm the bbls or parts back up and apply a coating of Mark Lee Express brown soln. Then immedietely into the boiling water tank as a normal Express Blue cycle.
5 or 10 min in the water, then out and card as normal.
For some reason after that one cycle with the Mark Lee Express,,,no more after rust from the L/Mtn.

The other that seems too simple,,is to complete the Slow Rust Bluing with the L/Mtn.
After final carding and while the bbl or parts are still warm,,wipe them down with a cloth patch dampened with linseed oil.

Now I happen to have been using what is called Refined Linseed Oil .
The stuff I use comes in small cutsey bottles for oil painting artists. The brand name is Windsor & Newton. There's other Art brands as well, probably the same? but this is what gave me results.
It may have additional stuff in it beyond what the big metal can off the shelf at Home Depot has. So perhaps that has something to do with it's sucess in this venture,,don't know.
I can remember using hardware store linseed (BLO) in the distant past as an after bluing treatment with no real success as far as combating after-rust.

I wipe the parts down and leave a thin even coating of the oil on the surfaces. The warm metal will make it an easy task to achieve. Then hang the parts for a couple days.
Yes the oil drys on the surfaces but will come right back off again with a followup wipe down with a clean cloth dampened with a decent gunoil. I use CLP as it's handy, but RemOil has also worked.
Very little black debris on the wipe down cloth and no after rust.