I know of someone who had a seedling that was always pale, so I'm inclined to also think it's just the way that one is. If it is some mutation that causes a problem with nutrients, the nutrient looks like it would be one that is not mobile in the plant because the older leaves are green and the younger ones are pale. I can't see from the picture if the chlorosis is interveinal or not but you might want to try giving it some iron. Some daylilies have issues with manganese but that's a harder one to experiment with than iron so I would do that first if you want to out of curiosity. When I wanted to test a pale daylily for iron deficiency I got a human iron pill and crushed it in water (didn't dissolve very well but it worked) and poured it over some of the leaves. Within two or three days those leaves had turned green while the rest of the plant was pale. If you have any iron for plants, though, that would obviously be better. If it's deficient the amount of iron in Miracle Gro etc. may not be enough. You could also plant it in the garden and water it with a teaspoon of vinegar to a litre of water once a week and see what that does (it frees up the iron in the soil - won't work with a soilless mix hence the garden). Another option would be Milorganite because that has 4% iron.
