Are they perhaps planted too deeply? I had several plants whose leaves were getting less and less green. I've dug them up and replanted them in the same spot but not so
deeply. They seem to be doing much better.

Yes, it can and a good dose of sunny weather should bring the green back. We went thru a very cloudy May and tons of water a lot of my daylilies yellowed. Half of June was cloudy and drippy as well. In the last couple weeks of hot sunny weather most have their healthy green color back. A few still have yellow foliage but they'll get better. I think it's the sun they miss. Thrips and spider mites can also give your foliage a dirty yellow look.

2020 what ? this post is almost 7 years old . Why is it being brought up again with absolutely no pertinent additions or questions?

Houzz competence.

I'm so glad it's still here, I'm asking the same question tonight about my daylilies!