Bridge and limitations for Dummy
From the Laws of Rubber Bridge:


  Law 42 – Dummy’s Rights
  Dummy is entitled to give information as to fact or
  Law but may not initiate the discussion.


So if declarer asks, dummy can say what suit was led. It looks like dummy can not do so without being asked, but I'd assume that if you explain the situation to your opponents (or a tournament judge), they'd allow you to dispense with the asking as long as you are consistent (e.g. naming the suit every time should be fine, but naming the suit sporadically might get you accused of some kind of signaling scheme).

Note that Law 42 for Duplicate Bridge is slightly less permissive; definitely clarify with your opponents first under those rules.