From my earliest gardening days I've noticed how one plant can make a neighboring plant look better just by color repetition. It seems to lead the eye to notice how the color is repeated and makes for a more pleasing look. I'll add a few examples.
The yellow lilies in the background seem to show up the center yellowish stripe of Firestar.
The echinacea, Bravado, repeats the same color as the nameless hydrangeas behind it.
Here the haze of light blue seems to infiltrate into the echinacea.
Daylily Siloam Toddler's ring of purple matches daylily Malaysian Monarch
The blue/purple of salvia Caradonna is the same as the outline of the eyelash on this pseudata, Shiryukyo, and the coreopsis Zagreb, in the background, plays up the yellow in the pseudata iris.
In 1994 Pamela Harper's book Color Echoes: Harmonizing Color in the Garden, was released and it promotes the idea of repetition of color. I had done it with daylilies many times prior without realizing exactly what it was that made certain plants look even better due to having the same color or tint in a nearby plant. It works equally well with foliage as with flowers.
https://www.amazon.com/dp/0025481851/
One of my favorites, Japanese iris Amethyst and friends.
Foliage friends:
Let's dig out those photos and post your most pleasing combinations.