I don't know what I did, but my screen colors are off base -- menu bar has a peach background and window title bars are something like lilac. Colors within any menu vary from blue to peach.
In System Preferences > General, I changed appearance and highlight color from blue to graphite, which changed nothing. But when I booted off a backup disk for some maintenance, the colors were as they should be.
My guess is a preference file is corrupted. Booted off the backup, I copied all the "com.apple . . ." preferences to my regular startup disk, replacing. Booted off the regular disk, no change. It must be another preference file, and I have no idea which one.
This is a 2009 iMac; OS is 10.11.6.