Ear Mites or Allergies?

Today as one of my cats was in my lap and the window shades were open I noticed that in her left ear there as a TON of brown buildup. It's dry and flaky  It didn't look gunky like the pictures I found on the internet of ear mite buildup.  I got some of it out but not sure what I should be looking for. Her right ear is fine. I've checked my 4 other cats and they are fine nothing brown in their ears.  The only reason I noticed the brown build-up is because she came and sat on my lap.  Her ear inside does look somewhat redish. Could be irritated.   My one male cat Smokie gets some brown build-up once in a while. I had brought him in and the vet said it was due to irritation by allergies and NOT mites. So is the buildup in the left ear from allergies? Lucky does have atmospheric allergies which there is really nothing I can do unless I want to put her on cortisone shots. Which they don't bother her much so I've left it alone.  But also wondering if it could be ear mites? I don't see any signs of her scratching her ears or shaking her head (unless I'm trying to clean that ear out).  She shows no signs other than one ear full of brown dry buildup.  My cats are indoor only, and they do not go outside unless it's in a carrier to be brought to the vet. And they go from house to car to vet and back. So just wondering if it could be allergies or if she somehow has earmites? Not sure how you can tell the difference? I know usually indoor only cats don't get ear mites, but they can get them.  

I also did switch cat litter recently trying a new brand. It was mixed in with the old too just so it wasn't an immediate change. Wondering if that could be causing the allergy and brown build up?  

 

In my experience earmite irritation is pretty itchy, and pretty obvious-  the cat will be shaking it's head a lot and scratching.  The buildup is also pretty dark.  My own kitty has struggled with allergies for the past five years (we control them for the most part with topical solutions from the vet and careful diet), and the first sign of a flareup is scratching at her ears.  There is no wax buildup though.

My best friend's cat has lots of ear wax buildup (it's light to medium brown) that the vet cleans out on a biannual basis using cotton balls and mineral oil (they live in the UK).  The vet thinks that particular cat just naturally produces a lot of wax.

Not sure if any of this is helpful, but it really doesn't sound like earmites to me.