I have a feeling MGM also trademarked the shoe design as well, which would likely make the red color too close. Many times with IP they do both, trademark and copyright, to cover their bases. Trademark does not expire as long as you continue to use it and continue to defend it.

So for example, eventually, inevitably, the copyright for Superman will expire, at least the portions of the character present in Action Comics #1. But since the S shield is trademarked, it will not be in the public domain. So you could technically adapt that original comic, but could never show the shield. I bet DC comics trademarked elements of the character design itself too.

I’m sure Disney holds some trademarks for visual elements of Mickey Mouse’s design as well.

Now I know some people will come at this and claim “that’s not how trademark works”, but it totally is. It’s not what it was originally intended to do, but that is what it does in modern America.

Three real world examples that show what you can do using trademark to its fullest:

I’m sure many people are familiar with those baby bottles that have the hole in them, which are supposed to be easier for baby to hold and prevent gas bubbles. So the original idea for that was patented, of course. And what was patented was the idea of a hole in a bottle. I saw the original drawings in class — it looked like a donut. Not at all what we think of. And that patent gave the inventor 20 years before it became available to everyone.

So then the inventor took the shape that actually worked the best and trademarked it, as a silhouette. And really it’s the only shape that gets all the benefits. And trademarks never expire, so every bottle in that classic shape is licensed from this guy.

Second example, sticking with bottles. The shape of the Coke bottle is trademarked. So even if they had patented the formula and now it was public (they never did so they never had to reveal it) you would never be able to bottle coke in the classic coke bottle. In fact, that trademark technically means you can’t re-sell those bottles either. Like you couldn’t refill them with homemade coke and sell them.

And last example I feel like giving: the Empire State Building is trademarked. Not just the name (of course that is) but the shape of the building. And some of the architectural details too I think. Because of that, if you want to use the ESB in a movie or tV show, you have to get permission and/or pay them. And any movie that features it will have at the end a little “used with permission statement”.

Trademark can be used to cover a LOT of things. It can make things really murky as some of these stories enter the public domain in the coming decades.