As an observation, I am in both a cold climate (zone 3) and an area which has long-tested as overall very acidic (Adirondack Mountains, northern NYS, due to acid rain). I have posted pictures of daylilies here that others have commented, 'Mine never shows that intensity of colour..."

An example: Guava Jelly, which is gorgeous here...


As opposed to other gardens, who post pics like these...


(comment by poster, this pic is too pink)

Not sure whether the vibrancy I 'always' see for Guava Jelly here in my gardens is the result of the acidity or the cold climate, but I do have other daylilies as well that seem to bear out comments of richer colours in cooler temps.

Cooler temps and acid soil. My blues and purples are really great and I think it's because my yard was an oak forest for thousands of years.

Having planted and/or moved most of my daylilies to an area that was occupied by Christmas trees it will be interesting to see if there are any color differences from where the transplants were growing previously, which was a bed over our septic leach beds. They did grow well there however provided a deer buffet. Have not tested soil in either area to date.