Just to follow up, again, on my own post, it seems what I had read and always believed (at least in reference to this particular type of coleus) was wrong.

More sun = more color.

...within reason. In my case, eastern/morning sun. I'm sure there are limits and I don't know what Full sun would do.

More sun = more color.

This has always been my experience with coleus.

You said in this case the plants are receiving about the same amount of sun. Is the sun exposure at the same time of day? Also, you said, "I later transferred two of them into different containers my girlfriend had earlier filled with *I think* a commercial potting mix. In a week or less, those two plants were a deep dark plum/purple while the others remained a somewhat faded green with tinges of pink here and there." This makes me wonder if the UN-repotted plants had used up all of the nutrients in their pots while the 2 'girlfriend/more colorful' plants had fresh nutrients in the "unused" soil.

Also, it looks like the more colorful plant is in the ground, where plants usually do best. And that's twice the more colorful plants have gotten fresh soil.

I don't know what too much eggshell would do, that's a possibility, too.

Action, Coleus are simple yet complex.

If they're set in a bright indirect sun, the colors are vivid, very pronounced, however, if placed in full sun, the colors fade..In summer, direct sun will burn the leaves within hours, even in a northern exposure.

When the sun is shining, and temps are high, Coleus confined to a pot need a lot more water opposed to one planted in the ground. Possibly 2 or 3 times a day.

A couple summers ago I bought a semi-large Coleus from an outdoor garden center. When I got home I placed it on the front porch. It was about 2pm.
Later that evening, between 5-6pm I went out front to take a peek..its poor leaves were curled and soil bone dry. (It was drenched when I bought it.) Talk about discolored foliage! It was originally Deep Purple with Gold edging. 4-5 hours after bringing it home, the leaves turned pale: Yellow and brown.

Action, you said both Coleus got the same amount of light? Are you certain one wasn't obstructed by a tree, building or anything elsee?
Was one repotted, fertilized, anything done differently?

It's strange..I hope I'm wrong, and probably am, but have you inspected the pale Coleus for Spider Mites? Coleus are easy targets. Like I said, hope I'm wrong, but it never hurts to check. Toni

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Curious how the cuttings are doing...?

Interesting that you ask. They did well.
At the end of the season (You'll note this was last year) I didn't get them all dug up in time and lost a few to frost but several were potted and overwintered in the house. They're now about 10" tall with lower leaves faded shades of pink, green and, due to the recent sunnier weather, the upper few sets brilliant red & crimson. I'm just about ready to chop them into pieces and start multiplying them again for this year.

Cool! I do that all summer and by the first frost, it usually looks like a coleus bomb went off in my yard. Any new observations from this summer?

Well, the two or three mentioned became a little over 30 new plants. While still in pots they were in full sun, and a nice deep, dark red/crimson/purple. Once planted in the ground they were more shaded so lost some of the color. These are now being pinched back with the prunings starting dozens more -- all doing well.

As for my original post, yes, I believe that more than soil nutrients, ph, moisture or anything else, sun exposure plays the largest role in developing deeper, darker colors.

There's information out there that would have one to believe that Coleus are fragile, shade-loving plants that'll wilt and die in the sun. From what I've seen that couldn't be more wrong.

Nice! I have that same red coleus in a pot on my front porch. I've been hesitant to take cuttings from it until last week because the pot is huge and it took a while for it to look proportionate. I totally agree with your last paragraph. The only thing I don't like about Coleus is the narrow selection at almost any store.

"...The only thing I don't like about Coleus is the narrow selection at almost any store."
..and that's one of the best uses for this and similar forums and why people here should take fuller advantage of them: to swap and trade with others with similar interests!

Especially with coleus type plants, it's so easy to propagate new plants from cuttings, trading with others doing the same with different plants, everyone could easily and inexpensively (just postage) expand the varieties in their gardens many times over!