I potted them, not in commercial potting soil but in my own mix (compost, vermicompost, soil, etc). I also included some ground eggshells because I read that Coleus prefer less acidic soil.

All cuttings were successful and I eventually had several, apparently, healthy plants. 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.

All plants are from the same cutting from the same plant, receive about the same amount of moisture and sun. (If anything, the two more colorful plants may be receiving slightly more sun even though my understanding is that less sun develops deeper colours). The only significant difference is the soil itself.

(I am now experimenting with the others but in the meantime) what is it that accounts for such a dramatic coloration difference?
The soil ph? Higher level of Nitrogen, Phosphorous, Potassium or Micro-nutrient? individually or in combination?

Previously, I had thought it was the amount of sun/shade alone that effected the depth of color in Coleus. Obviously it's something more ..but what?

Thanks

Reading further makes it appear that what I had earlier believed about shade causing Coleus to develop deeper colours is incorrect.

Watering, Fertilizing and Pruning:

Coleus prefer their roots to be in soil that is evenly moist but not soggy. The more your coleus are exposed to sun and wind, the more often they will need to be watered. Daily or twice-daily watering may be necessary, especially as the coleus mature and develop a large leaf canopy. Coleus that have wilted will usually respond quickly when watered, but coleus that have been repeatedly allowed to wilt will quickly show signs of wear and tear that make them less attractive: crispy edges, dry brown spots on the leaves, bleaching, and dull color. Mulch is recommended for Coleus grown in the ground in hot areas. Drip irrigation is great for plants grown in pots or in the ground because it keeps the water off the leaves. Watering is best done in the early morning so the leaves have time to dry before the sun hits them. Coleus leaves should always be dry by the time darkness falls to decrease the chance of fungal disease occurring.

Coleus do not show their best color when over fertilized, so fertilize at half strength on a regular basis, or use a time release fertilizer (recommended).
Pinching the growing tips of your coleus when they are young will increase branching and make your plant more bushy. Some coleus plants will bloom, and the blooms may be pinched off or allowed to develop according to your personal preference. It doesn't hurt coleus to be pinched and pruned. In fact, it is often necessary to prune coleus in mid-summer to help the plant keep its shape and freshen its appearance. A plant that is allowed to overgrow may cause its pot to topple or branches may snap under their own weight.

Sun or Shade? The amount of light your Coleus plants receive will affect their coloration, so the same variety of Coleus grown in two different locations in your yard may vary in appearance. Most Coleus can take some sun, and several can tolerate full sun if properly acclimated. Coleus grown in full sun will create more pigmentation to protect itself from the sun. For example, bright yellow or chartreuse may become a dark gold or green. Oranges may become red. Reds may become burgundy. Dark colors may dominate lighter colors on the leaf. Results also vary according to region. Full sun for our friends in Texas or Florida is much stronger than full sun for us here in Michigan! The most important thing to remember when growing coleus in full sun is that they need proper preparation and consistent watering to look their best.

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.

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.

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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