1 Eleven: It looks like we need to go through that archway at the end of the hall.
2 Dustin: No! You never go through the obvious doorway in the Tomb of Horrors!
3 Eleven: We went through that previous archway that looked just like this.
4 Lucas: You never listen to other people’s advice when going through the Tomb of Horrors!

This looks like the same misty portal seen in #4753 for a very good reason. No, it's not actually the same portal, but it is represented by the same image in the adventure's illustration booklet. For the first portal, the glowing stones around it are described as (clockwise from bottom left): yellow, blue, and orange. For this portal the stones are described as olive, russet, and citron. I kid you not. I had to look up citron to find out what colour that was.

While searching, I discovered that citron is often found in contexts associated with russet. Curious...

Being interested in colour science, I looked a little more closely and found that citron and russet are often used as colour terms to describe tertiary colours in the traditional artistic red-yellow-blue colour model. Under this model, the tertiary colours listed in Wikipedia are slate, russet, and citron - although it notes that sometimes (according to two cited references) olive is used instead of slate (or, confusingly, instead of citron).

I managed to find some other sources that treat [olive, russet, citron] as a set of associated colours:

Tertiary colour exercises by an artist.

Text from the Cyclopedia of Painting by George D. Armstrong (Frederick J. Drake & Co., Publishers, 1908), which uses "citrine" as a variant spelling for "citron" (and which is not one of the two other works cited by Wikipedia):

Tertiary colors are three only, Citrine, Russet and Olive. Each of these is composed of, or can be resolved into, either two secondary colors or the three primaries; thus Citrine consists of Green and Orange, or of a predominant Yellow with Blue and Red; Russet is compounded of Orange and Purple, or of a predominant Red with Blue and Yellow, and Olive is composed of Purple and Green, or of a predominant Blue with Yellow and Red. [Section on Primary Colors.]

So there's reasonable evidence that Gary Gygax, the author of Tomb of Horrors was deliberately using a set of three tertiary colours from some reference he had at hand. It's very much in his character to use things which have significant connections like this.

But this raises an interesting question. Why for the first portal are the colours specified as [yellow, blue, orange]? In the same red-yellow-blue colour model, yellow and blue are considered primary colours, while orange is secondary. The other secondaries are green and purple. Even if we allow use of other colour models, such as the red-green-blue of light mixing or cyan-magenta-yellow, we never find [yellow, blue, orange] at the same primary, secondary, or tertiary level of mixing. So it seems an odd choice for a set of three colours.

But now I wonder if this was actually meant by Gygax as a deliberate clue. The first (yellow-blue-orange) portal was safe to use (if you pressed the glowing stones in the correct sequence), whereas this (olive-russet-citron) portal is (spoiler alert^[1]) decidedly not safe. The fact that one portal was adorned with a non-consistent set of colours, while the other bears a very specifically consistent set of tertiaries could quite plausibly (given the way Gygax's puzzle-driven mind worked) be an intended clue to players that this portal is different to the previous one in some important way.

Some Internet searching turns up several discussion about the Tomb of Horrors that mention the olive, russet, and citron stones, but they are mostly just read-throughs, reviews, or table logs of actual play. Some of these comment about how Gygax loves to use obscure words and cite these colour word choices as examples of his overly arcane vocabulary. But I can't find a single discussion of the fact that these colours are specifically a set of tertiary colours, or that it might be intended as a deliberate clue.

Good lord. Am I the first person in 47 years to have discovered a previously unknown clue that Gary Gygax put into Tomb of Horrors???

Later thoughts: I just noticed that the initials of [olive, russet, citron] in that order spell ORC - the name of a famously evil adversary monster in early editions of Dungeons & Dragons (likely based on Tolkien's stories). That could be another clue that this portal is dangerous. Annoyingly, the initials of [yellow, blue, orange] in the correct order don't spell anything, though they do anagram to BOY. It feels like there's just enough pattern here to mean something, but it's still not clear if any of this really is deliberate or not.

I should also, in fairness, point out that there's another, possibly easier, way to determine that the (yellow-blue-orange) portal is safe while the (olive-russet-citron) portal is not. Experimenting by pressing the stones in various orders will reveal that the yellow-blue-orange ones are pressed in that order, the mist obscuring the portal clears, inviting brave adventurers to step through it. In contrast, no combination of pressing the olive-russet-citron stones will clear the mist away - which really should be enough of a clue to warn anyone against entering.

[1] Not that it's much of a spoiler to say that anything in the Tomb of Horrors is not safe.