What can I do to get my daylily plants to be a dark green instead of light green/yellow looking?
I am located in zone 8 in southwest Georgia!

Some daylilies are naturally a lighter green but otherwise it could be a soil pH issue and/or a nutrient deficiency. Are the leaves pale all over, or just between the veins? Are some leaves paler than others, if so is it the youngest or the oldest. Have you fertilized and with what?

They are pale all over. They were planted several years ago so they are about the same age but some plants are pale and some are dark green. I had the soil brought in by a seller and I have suspected it might be the soil. Should I have soil samples evaluated to determine what it needs?

It's normal for there to be some variation, some cultivars are naturally a darker green than others but if the ones that are pale now were darker green before or if you have some plants of the same cultivar that vary in colour then yes, best to get a soil test and find out exactly what's going on and what to do. Have they ever been given a fertilizer with a good amount of nitrogen in it? Have they had sufficient rainfall/irrigation?

I love the dark green foliage as well, and in my little attempts at hybridizing that is one thing that I would like to have my plants display.
I am so proud of a cross I made, even though it has a plain Jane bloom, the foliage is beautiful, so something to work with in the future.I do find that sometimes adding milorganite can help green up plants, but it is really mostly a cultivar thing. Some plants just have light foliage and others darker foliage. Some daylilies have narrow leaves and some plants have very broad leaves, I prefer the dark green broad leaves that resist rust and leafstreak.
The leaves really should not be yellow looking, I made the mistake of mixing my rust spray incorrectly the last time I used it and it has caused my daylilies I sprayed it on to turn a lighter more yellow looking color. Hope that chemical spraying has not caused your problem.
Maybe improving the soil with organic matter, and a soil test and make sure they get plenty of water.

Thanks so much for the replies! I had a soil sample done and my soil had a low pH. Also it needed lime! After the blooms were gone I put 13-13-13 (I planned on using 10-10-10 and could not get it at time so I took a chance on 13-13-13!)and lime. It has made such a difference in my day lilies!!! They are dark green again look so much better!!We have had good rainfall this summer so that has made a huge difference!! I will post a photo of my bed when they bloom again!!! Thanks again!! It so nice to have gardening friends!!!!!!!!!