(setq rainbow-identifiers-faces-to-override (default font-lock-variable-name-face))

(defun rainbow-identifiers-face-overridable (begin end)
  "Test if all the faces under the identifier are overridable."
  (let ((current begin)
        (contains-non-default nil))
    (catch 'rainbow-identifiers--face-overridable
      (while (progn
               (let ((face (get-text-property current 'face)))
                 ; In PHP function names have no style
                 (if (not (or (eq face 'default) (null face))) (setq contains-non-default t))
                 (cond
                  ((null face)
                   t)
                  ((listp face)
                   (dolist (face* face)
                     (unless (memq face* rainbow-identifiers-faces-to-override)
                       (throw 'rainbow-identifiers--face-overridable nil))))
                  (t
                   (unless (memq face rainbow-identifiers-faces-to-override)
                     (throw 'rainbow-identifiers--face-overridable nil)))))
               (setq current (next-single-property-change current 'face nil end))
               (not (eq current end))))
      contains-non-default)))