comparison of winter leaf coloration of different citrus varieties

[SoCal2warm]

The following three varieties were left outside over the winter, from left to right in the picture: Reinking pomelo, Satsuma, Yuzu.

Notice the hue of the leaf coloration on each of them, there's a clear pattern.
The leaves on the Yuzu are the most green, while the leaves on the pomelo are the most yellowish. The Satsuma, perhaps not surprisingly, is intermediate in leaf color.

I actually have more than one of each of these, but they would have been too much to all show in the picture. They all look consistent within each variety. (So this experimental trial wasn't just one single specimen of each variety)

Obviously the leaf coloration is in line with the expected hardiness level of each of these varieties (with Yuzu being the most tolerant to colder temperatures out of the three).
I think this comparative picture of leaf coloration is very indicative of different levels of hardiness of these different varieties.

they are seedlings growing on their own roots

(Olympia, WA, zone 8a)

[Millet]

Very interesting. Thanks

[Ilya11]

But where is a picture before you put them outdoor?

[lebmung]

they are seedlings growing on their own roots

Nice experiment! 
Notice something yuzu has strong roots, used as a rootstock in Japan, not immune to root rot but somehow tolerate it to a certain extend.
Satsumas are very sensitive to root rot. If the satsuma was grafted on PT it would have been green.
The reason that is has that colour is the root damage so poor water and nutrients uptake.

[SoCal2warm]

Notice something yuzu has strong roots, used as a rootstock in Japan, not immune to root rot but somehow tolerate it to a certain extend.

All the seedling containers were left under a glass patio table to prevent the containers from being flooded with rain, since the winters are very wet here.
I was not the least bit concerned about them drying out because the humidity levels in the air are very high in winter, in this climate, with rain drizzle frequently in the air, and the constantly very cool temperatures pretty much preclude evaporation. They have not been watered the entire winter, and they did not need to be. The soil has remained moist but not waterlogged. 

[SoCal2warm]

Here's a picture of three seedlings from taiwanica x trifoliate, so you can see how they compare:


also left outside over the winter, same conditions

[lebmung]

You do realize that transparent containers inhibit root formation and promote algae growth.

[SoCal2warm]

You do realize that transparent containers inhibit root formation and promote algae growth.

Transparent containers are what I had.

At least it allows one to easily be able to see the moisture level in the soil.

[lebmung]

You do realize that transparent containers inhibit root formation and promote algae growth.

Transparent containers are what I had.

At least it allows one to easily be able to see the moisture level in the soil.

it would be wise to paint it black with an acrylic paint, but that takes time.
I tell you one fast cheap method, buy black garbage bags put it around a with scotch tape, they will last one year. the heat from now in the spring is essential for the roots. And during the winter put aluminum foil if the sun it's very strong.

[Sunmicroman]

Looks like you're dealing with leaf chlorosis (yellowing), very common problem with subtropicals grown in the Pac NW due to the cold dampness a lot of the year. I don't think cold hardiness of a plant has as much to do with it, as does a citrus plant's ability to tolerate cold "wet feet" (which none like, but some tolerate better than others). You will see this issue on Trachy palms (as well as other palm species) from time time in our climate too. I have found over the years that cold dampness is much more of an issue than outright cold hardiness where we live. That's why finding specimens that can tolerate the cold dampness (including soil temps/moisture) in conjunction with cold hardiness is the real key to success with cold hardy citrus here. I would try possibly amending your soil with some sand for drainage and so it can dry out faster. I think container plants are much more susceptible to this issue unfortunately (I have grown many many over the years).