I appreciate both responses, but apepp employed some techniques I was not as familiar with. My source picture actually was a bit more complicated than my question described. The door is white and its trim is a color. I wanted to replace both with a single new color that I found at Sherwin-Williams paint company's site, while leaving the wall color around these items alone.
I didn't understand everything apepp did but it gave great hints. I selected the door in a duplicate layer of the original image, inverted it, deleted everything but the door, used Levels to make it as white as possible while retaining the details in the door trim, then applied a Layer Style | Color Overlay to it. I changed the Blend Mode to Multiply, which restored the trim details, then clicked on the color and pointed the eye-dropper at the Sherwin-Williams paint sample in another window. That got the door color very close to the paint sample.
I wanted to repeat this process on the trim but I first had to remove its old color and get it as white and detailed as the door. I used Levels adjustments to accomplish this, then repeated the Color Overlay steps above.
Again, thank you to both respondents for the help. I hope this resolution helps someone else in the future. Before landing here I found many other people complaining about unpredictable results when using Photoshop's color replacement tools (mostly just yielding tints of a color or a muddy gray).