Search field above music library has additional options that can be added before the search words. The default search (without any) is by default (E.), it can be changed in Preferences > Misc.
Either one letter or two letters followed by a dot
Search for every word,
E.purple green would find a song "purple and green song" but not songs "purple song" or "green song".
Search for any word,
A.purple green would find songs "purple and green song" and songs "purple song" and "green song".
Search artist field only, if any of the words are found.
Search artist field only, when all of the words are found.
Search title field only.
Search 'Folder' field only.
Search 'Album' field only.
When no option has been added, phrase can be searched instead of words when the words are quoted:
"Purple and green"
Search field has autofill feature that tries to guess what you are typing. Multiple possibilities can be navigated by clicking 'return' key. This autofill feature can be disabled or modified in preferences > Misc.