2nd plant, which IMO is more yellow

Since the underplanting is lavender there could be an implication that the potting soil used is too lean for citrus. And maybe also too alkaline. In the meantime I don't know about this particular type but most kinds really aren't year round outdoor hardy plants in the Seattle area - there is a reason why very, very few examples of much size and vintage at all are ever seen here. All the more so because yours are in containers they should definitely be moved under cover any time temperatures below the 30s F. are predicted.

Yuzu and its hybrids are possibly some of the most cold tolerant varieties of citrus. Local vendors rate them to zone 7. But if in a container I'm not sure I'd risk extreme PNW cold without protection.

btw, the fruit is typically harvested while still green. And container grown plants need routine fertilization.....I'd suspect that has something to do with the poor color.

The yellowing of older leaves on citrus indicates nitrogen deficiency. It can also mean that the potting soil is alkaline (citrus likes acidic soil) or overwatered.

All plants in containers need to be fertilized, but especially plants that produce fruit or abundant blooms. You can try spraying your citrus with fish emulsion (or fish and kelp emulsion). This is a fast way to green up yellowing leaves. Stinky but effective for container plants.

Speaking from experience, I've had no luck keeping citrus healthy in pots, despite careful watering, fertilizing, good climate, etc. They can be very temperamental if the conditions are not ideal. I would say growing it in Seattle is definitely pushing the envelope.

Thanks for the info. This particular variety is hardy to my climate and is why I picked it since I didn't forsee myself moving citrus inside to overwinter (as much as I love it). I didn't realize this would do better in the ground vs a pot for my climate so sounds like I need to transplant it or just see what happens this winter (and possibly lose it).

Many said I need to fertilize it...with what though? Now that some mention it I did apply kelp fertilizer a few times this summer but it's been a while.

Also the fruit is firm with no give, should I just pick now anyways?

Hardiness can be tricky -- it means the plant won't be killed during your average winter, but it doesn't mean it's going to thrive in your specific location. When you keep something in an exposed pot, you should expect your hardiness to go down at least one point, which would put you at Zone 7.

For fertilizer, you can go on amazon and find something that's formulated for citrus. I used to use Foliage-Pro (by Dyna-Gro) that a lot of growers recommend. Follow instructions on the package.

Keep in mind that, in Japan, Sudachi are usually picked green and used like a lemon.

You probably could allow it to ripen more and turn orange, to get a somewhat sweeter fruit, but by then the flavor of the fruit would be headed into a little bit of more of an insipid direction, with less sourness and flavor.

I do have a Sudachi plant that has survived outside this past (2019-2020) winter, unprotected, an hour south of Seattle in Olympia. It has not done as well or grown quite as vigorously as Yuzu, but it has seemed to survive okay, if not really doing very well.

You want a tip for this climate? Keep the soil moist in Summer, and keep the soil from getting too waterlogged and oversaturated with water during the rainy winters. Citrus roots don't like cold combined with sitting in stagnant water.

In general, other factors will make far more of a difference than fertilizer.

From my experience, it's not unnormal for citrus leaves (of hardier citrus varieties) to look like that during the winter. The leaves will probably not drop off, and they will stay alive into next year, but next year the tree will grow new leaves. The leaves start becoming less healthy looking in the cold. They can still be alive though. I've noticed some of the leaves start recovering a healthy green color once summer starts.

In fact, with the exception of Poncirus trifoliata which can be deciduous, you can get a good indication of how hardy a particular variety of citrus is by how green the leaves stay in winter. The less hardy the variety is, the more yellowish the leaves will turn. (This is assuming there are not waterlogging root issues causes the leaves to turn yellow)

The above posts are correct, your tree is very under fertilized. For container grown trees do not use organic fertilizers. As JoJo wrote Foliage Pro is a good fertilizer for container production.

Yellowing leaf is not a sign of nutrient issues in this climate. Remember, she lives in Seattle, and it's getting into winter right now.

Fertilizer won't solve the leave coloration issues. The leaves are turning yellow because the temperatures are cold, roots stop working well at lower temperatures, and the plant can't draw up nutrients like iron.

The best thing is to just let the plant be. Don't try to heat it, it will make the plant begin coming out of dormancy and make the plant more vulnerable and unable to deal with cold later.