I bought two t206s that had around 40 years of light exposure. Red faded for sure, the other colors didn't.
On one there's still some gloss where the red was, on the other it's not visible.
The individual ink layers aren't all that thick, so a faded card won't always show the ink layer as a clear layer. (For lithography, other forms of printing might show a layer with a visible thickness. )
Fading of inks is a tricky thing, especially in that era where natural pigments and dyes were being replaced by synthetics. What's common with T206s is that the bright red will fade often completely while the underlying pink layer won't fade. The bright red is probably cochineal which produces a brilliant red, but is only a bit more lightfast than reds made from plants. The pink is probably one of the two early synthetics that didn't produce a brilliant red, but were far more lightfast.
Another trait of a real missing color is that most of them are missing another color as well.
Downey and Lobert are the two known faded cards. The lot they're from is probably the one that made SGC abandon slabbing cards as missing colors. There were a few lots from the same seller, some fairly large and still on the backing. All mentioned the cards being framed and in sunlight for around 40 years. I bought one lot early to see if they might actually be missing colors, and decided they weren't another buyer bought one of the bigger lots and a few of them passed as missing colors. It was discussed here, and shortly after someone sent another more likely card to SGC and found they no longer slabbed cards as missing colors.
Beck is a genuine missing color, missing both pink and gray. Subtle but a cool card.
Huggins is an odd one. A few subjects are found with a very pale look. I believe they might be missing pink, and possibly another color, or might be examples of cards missing pink, but not having the second color as part of the design (Like maybe gray)
6-8 colors is the minimum, some cards have 10 or more.
Steve B