How Big Is That Niche Anyway?

Sometimes I justify having endless variations on a particular shade (like taupe) by saying that the finish or the tonal differences are distinct enough to keep them all.  This also helps make room for any new brands I like to squeeze into my taupe hoard.  Also when all the makeup I own got to be a certain size, it seemed that I needed to do some mental gymnastics to make reasons to keep everything.  I tend to purge on a regular basis, but I also tend to go through periods when I get particularly obsessed with something specific - like bright pink lipsticks, a perfect lilac nail polish.  And then while I was doing the Suqqu Mitsucha post over the weekend, I was hearing some alarm bells dinging.  I was originally musing that Tom Ford Cognac Sable and Mitsucha were if not dupes, awfully similar.
Looking at the palettes a bit closer, they are definitely not dupes.  There some definite similarities in tones, but both the finish and actual color to color comparisons show they are not the same.
Both are warm palettes for sure.  Cognac Sable even more so and even more pigmented than Mitsucha. The dark cool brown in Cognac Sable was stuck in my faulty memory as a cool deep matte shade, but it actually is generally satin with some distinct shimmer bits.  Mitsucha is slightly cooler in comparison, has an overall golden cast, while Cognac Sable rings more copper.  I do quite love the yellow gold of Mitsucha.  That is definitely something I don't own in any of my other eye shadow shades.

Here's a look at the different finishes of the two palettes.
The coppery glitter shade nearest to us has some serious serious bling.  Mitsucha overall is a more sedate quad.

So having had a closer look at the two, I seem to be making the mental argument that these are both worthy keepers.  Right?  For those with largish makeup collections, do you sometimes feel like when products are applied, they seem awfully similar?  Either I am feeling particularly like purging stuff, or I am at a point where I want some really BIG distinctions among my products.  Does Cognac Sable and Mitsucha feel different enough?  These are philosophical questions of the deepest, most though-provoking kind. I know.  I've blown my own mind.

Is there some kind of a formula that calculates that question and comes to an answer that says to keep or  get rid of something?

I always was terrible at math.

Visée Blend Color Cheeks in RD-2

Suqqu Blend Color Eye Shadow in Mitsucha 15