Some of my best, brightest color daylilies are:
Baby Betsy, a very clear pink that attracts the eye without being garish, and beats the heat and sun, staying pretty close in color all the way through the day. Not a huge amount of blooms, but enough to keep the garden nicely spiced up:
Entire plant, morning color, and early evening color after 100 degree full-sun day
Beaufort Slim Pitkin is vibrant, big, and really anchors the garden with its good form. A steady bloomer.
Full plant (the scape in front with partly opened bud) and 3 pm photo from 100 degree day - color virtually the same as morning opener, though I should note it is blooming in full shade.
Answering Angels has been a very strong garden bloomer with lots of buds, and the mix of purple and cream is great for punctuating solid color in the garden. I enjoy that each blossom is fairly consistent in pattern, but may have some beautiful variation all its own, too. Very good color retention in full/hot sun and haven't seen any "burning" (no very dark spots or crumpling up) during day of bloom. Photos show full plant, normal morning color, and close-up of color after a day of 100 degrees full sun in my garden.
Just a few that come to mind. Have you found some favorites in your garden?