If you have remembered the journal that I posted that I visited the Charles M Schulz Museum/Research Center in Santa Rosa, CA: https://www.furaffinity.net/journal/10708638/ , you would see that there is the response that I posted at the end of the journal. If you ever see the gift shops there, Snoopy has a black collar and also there is a red-collared Snoopy as plush. As an active follower of Snoopy in the past and seeing the holiday specials, there is that question that would beg the avid fan of the notable black and white beagle that Charlie Brown owns, and regularly his friends meeting him.
As it is that Snoopy is a dog constantly having his collar on, and that there is no indication ever that he ever takes it off to get cleaned or have others to wear for other occasions, one would default that he would wear one colored collar. Herein lies the problem-on some regular Sunday strips, on the same setting and same day and even when he does move, the color of the collar sometimes change! It is a strange error that Schulz would do.
So I did ask the researcher this:

"As Snoopy has a collar, he does wear black, but I see red as well. Which color is correct, or which color collar he wears the most?"

Now if you do see The Snoopy Show on Apple TV: https://tv.apple.com/us/show/the-sn.....d=tvs.sbd.4000 , or even the The Peanuts Movie: (Youtube snippet) https://www.youtube.com/watch?v=YubVPL1X_AQ you will see that his collar is dark red. On the specials like "A Charlie Brown Christmas" (snippet:) https://www.youtube.com/watch?v=YubVPL1X_AQ , you will see it black. But what will turn it upside down is the response that I got from the Museum/Research was this from the archivist today that goes by the name, Sarah Breaux:
"Snoopy's collar is normally black, likely due to to the day strip being printed for many years in black and white."

WELL... good grief!

It could be that once Schulz died in 2000 and that nobody wishes to continue the Peanuts strip, my guess is that the animators who are doing the new shows and movies can have the leeway of giving Snoopy the change in collar.
I do have a hypothesis-since there is this fact that Schulz and his family do not wish to have the strip continue on, they would not worry about such backlash from the estate of Schulz. The collar then would totally be of Schulz's creation and it is...

BLACK.

For as long as Schulz lives, and I have seen a video of him talking at the Center in the 90's, and it seems that he would be the one dictating how everyone in the Peanuts shows and movies should be, and he would have Snoopy be according to how he wants him to be. Well, now he is in his grave, and for some whom have seen Snoopy in red on the small errors that the printing services would do when they put color for Sunday, they do capture the red collar. Wherewith, Snoopy is now having the red collar.
Now, one will see the artist here- bradsnoopy97, it would be that when he sees the more recent specials that the animators have done, the beagle AFTER Schulz's death would have his default collar red. It could be then that the 2000's are long-gone, and the creators and he would like to have more color to Snoopy, and they will have his neck be colorful-there would be no say against the estate nor the research center because the rights to having Snoopy have been tranferred to the media studios- Wikipedia says that WildBrain (Formerly DHX media) and Sony own 80% of licensing, and so they would therefore have the final say on how the characters are when Peanuts goes to screen, or to a play. So for the Schulz Museum and the Yokohama Japan's The Snoopy Shop: https://en.wikipedia.org/wiki/Peanu.....n_Yokohama.jpg (I think there is a museum there too. But do note the large Snoopy statue in the pic-red collar...), after the last movie in 1980, Bon Voyage, Charlie Brown (and Don't Come Back!!), it will be the last you see of Snoopy being in a black collar at a cinema, and in 2011 it was "Happiness is a Warm Blanket, Charlie Brown"; so from hereon starting with the 2021 special, "Snoopy Presents: For Auld Lang Syne", it is red in this millennium. So artist BradSnoopy97 would be correct in the 1000 years we'll have to have Snoopy in a red collar- it is thanks to that small error that the Sunday strip printers responsible for putting color to the strip that you would sometimes see.

So if it is that you will buy such merch of Snoopy, if you are true to what Schulz has done, you would go for the black collar. Otherwise, if you want to be with change and you would go according to what the media has done and to go and be incorrect, get Snoopy in the red collar. It's not anytime soon that his collar will revert back to red, but it could be soon that we'll see the last of the black one as in mainstream, it is red, but historically for 50 years before his death, it's always been black.

Well, hope that helps out in this question that has been bugging the true Peanuts fans. No, don't use my journal as a reference (DON'T PUT IT IN YOUR RESEARCH PAPER!), but it is up to you on what you want in some Peanuts characters. I'll go for black since that is what I grew up with while reading the strip.

I will go and see that place again, so yes, if you were to see me with Snoopy merch, it will be him in a black collar if I go to either gift shop.