Here is an example of how to color search results based on tags.
(the following text would be placed in your ~/.emacs file):
- (setq notmuch-search-line-faces '((\"unread\" . (:foreground \"green\"))
+ (setq notmuch-search-line-faces \\='((\"unread\" . (:foreground \"green\"))
(\"deleted\" . (:foreground \"red\"
:background \"blue\"))))