Overwatering can be one cause of yellowing leaves (or leaves with a sickly yellow-green color that never improves). Are there areas where the plant has been grown thus far under any type of automated irrigation system, or are they being watered often or do they collect runoff water from another area, such as a roof downspout outlet or gully?

Hellebores are more drought-tolerant than wet-tolerant, at least  once they're established, although getting too dry in the first year or so after planting could also damage roots and make recovery difficult for the plant. Yellowing or off-color leaves suggests a root health problem, especially when the entire plant is a consistent off color and not just a couple individual leaves.

It's hard to say if a nutrient deficiency issue is involved without having the results of a laboratory soil test, which measures levels of most major nutrients and the soil's acidity (pH), since pH level impacts how well roots can absorb nutrients. (DIY soil probes and at-home testing kits are not as accurate or easy to interpret as a lab test.) Usually, Hellebores would not need to be fertilized, and soils are not typically deficient enough to need supplementation in the form of fertilizer. If the pH is too far outside of the ideal range for Hellebores, then that might need correcting (or the plant moved to spot with a better pH level, which would be easier than making adjustments every few years). Hellebores aren't too picky about soil pH, so it would probably have to be quite far into acidic or alkaline conditions to be having this much of an impact, however. References vary a little on the recommended pH range, but it's generally in the 6.0 to 8.0 range (7.0 being pH-neutral).

The yellowish tinge to the foliage doesn't look like chlorosis, which is caused by leaf deficiencies of iron, manganese, magnesium, or zinc. It doesn't necessarily mean that the soil is deficient in any of those nutrients, but that the roots aren't absorbing enough, possibly due to a pH issue instead. Or, it could be from a poorly-functioning root system, which can stem from root rot (if overwatered) or some other infection or stressor. Low nitrogen can also cause leaf yellowing.

If the roots are not functioning well for some reason, then no amount of fertilizer supplementation (most well-rounded fertilizers will contain the micronutrients that include iron, manganese, magnesium, and zinc) will help. You might want to examine the roots when you are ready to move it again, since its winter hardiness will be less reliable if kept in a pot for the winter. (Other than cold exposure, another big risk if the pot getting too dry.) If you find roots that are mushy-textured and which pull apart fairly easily, where they separate into an outer layer and a hairlike or stringy inner core, that suggests root rot set in at some point. If the roots are instead firm and take more force to snap cleanly in two, then something else is probably the cause of decline.

Hellebores do best in partial shade (perhaps 4 hours of hot sun or less, although it's not concrete) to shade. They can tolerate some direct summer sun, but might lose their darker green leaf color and become yellower green instead.

We can't identify the exact cause of its discoloration from the photos, but if the roots look to be in good condition, then planting it in a well-drained spot that gets shade or dappled light during the hot part of the day in summer might spur recovery when growth resumes next year. It's possible the root system started out in poor shape and just never improved, so the plant declined and is stagnating instead of growing and looking better. If that is the case, it may be hard to get it to recover, especially since it's hard to guess why it started to struggle in the first place.

Miri