I haven’t supplied much description of my main character, mainly because it didn’t come up naturally. Her parents are described, so you can get a basic idea, fair skin, light eyes—but I never list any of her physical traits directly. But later, about six chapters in, another character mentions that she has red hair. Will this disrupt the mental image that readers have already formed of her? Should I mention it earlier, or not at all?
As well as describing the red hair early as the other answers suggest I'd also advise describing it explicitly. Don't feel obligated to add in all your character's other physical traits as well, if the red hair is the only trait that is relevant make it the only one you describe. Otherwise the reader may not pick up on it.
For example I was reading a story where early on the main character was described both physically and non-physically in successive paragraphs.
The physical description was a paragraph describing everything from their hair length to the kind of jeans they were wearing and it never came up. No other character every reacted to any aspect of it and it was never considered by the main character at all.
The non-physical description however consisted of just saying the main character was very confident while everyone else thought they were arrogant, and everyone in the story responded to that in some way. So that was the description which stuck with me and that's what I based my mental image of the character on and for me that included the character having a big beard.
This meant that half way through the story when the character put on a false beard to infiltrate some crime organisation it really threw me. I went back and checked and in the middle of the physical description it did say they were clean shaven I'd just completely missed it as it was buried amongst all the other details and had never come up since.
In my case it didn't matter as the false beard wasn't important, the fact they were disguised was, and in fact after saying it had been put on the beard itself never came up again. But in your story if the red hair is important make it the focus when you describe it so the reader definitely picks up on it.
Do it early, if at all. People usually form their impressions of a character based on the first time they see them enter the story, and if you don't describe them there the image won't stick in their mind.
But later, about six chapters in, another character mentions that she has red hair. Will this disrupt the mental image that readers have already formed of her? Should I mention it earlier, or not at all?
Yes, you must mention it earlier than this, because this is the point where it become plot-relevant. Much like a mystery, all the pertinent information must be established beforehand to make a plot development seem natural rather than shoehorned in. I.e., if the character has red hair, but for the first six chapters the reader assumed she had black hair, then readers will be completely lost when you mention how the character's red hair is important.
This is even worse for physical traits than mental ones, as physical traits are something that is immediately obvious upon first notice. The only time someone would not make note of these things is if it was in a context that no one would notice: say everyone in the room including the viewpoint character had red hair.
A good example for this was a case I saw where a friend who was reading the Animorphs books accidentally started with one that did not describe the Hork-Bajir, and based on the book's mention of blades accidentally thought the Andalites were the Hork-Bajir. When they later picked up the Hork-Bajir Chronicles and saw they were anthropomorphic Stegosauruses they were very confused.
For many people, the story plays out like a movie in their head as they read. If you don't "cast" a face into the role quickly, they'll have already filled it with their own imagination. Within a few sentences of introducing a new character, stories tend to describe the important characteristics you'd notice if you saw them on screen to avoid pulling the rug out from under the reader by revealing relevant details too late.
There are some stories where the appearance of the main characters is withheld from the reader until much later. This is done with a purpose: the reader makes an assumption about the appearance of the characters. Maybe they have red hair, maybe not and then — boom — they're actually aliens! (This in particular is an allusion to Asimov, but there are dozens of stories like that, some about bigger twists and others about more minor details like yours.) This only works because there's a shock factor involved.
Here is the worst example I could think of where a writer omitted mentioning a detail of their protagonist's appearance until that detail became important to the story, thus making that detail seem like a deus ex machina.
There is a famous science fiction story "Deadline", by Cleve Cartmill, Astounding Science Fiction, March 1944, about wartime espionage and a project to build a fission nuclear bomb. Since this was during World War Two when the USA and other countries had top secret projects to build fission nuclear bombs, government agents investigated to see if there was a leak from the Manhattan Project, but were convinced that it was based on publically available information.
Here is a link to an article about the investigation of the story:
The story is set on the planet Cathor, so the characters are extraterrestrials who aren't human but seem more or less similar to humans in the story. Many readers might assume, for lack of descripion, that the Cathorians probably look exactly like Earth humans. The protagonist, Ybor Sebrof, is captured by the enemy. And near the end of the story, the protagonist, while a prisoner, has an enemy pointing a gun at him.
The doctor chuckled. "Why, you are telling the truth."
Dr. Sitruc relaxed, and Ybor moved. He whippped his short, prehensile tail around the barrel of Dr. Sitruc's gun, yanked the wapon down at the same time his fist cracked the scientist's chin. His free hand yanked the gun out of Dr. Sitruc's hand.
And that was the first mention of Ybor's tail - and presumably the tails of all the other natives of Cathor - in the story, despite how "handy" Ybor's tail turns out to be.
And if a writer doesn't want their story to be ridculed as much as "Deadline" was for Ybor's magically appearing tail, they should mention all the details of their protagonist's appearance which are important early in the story, before those details become important.