I'm alive, some times

Traits in general can be sorted onto a spectrum: they evolved because they are in some way advantageous for survival and reproduction, or is so neutral to evolutionary success it just never needed to be selected out.

Eridians don’t percieve colors, so different color carapaces probably lands closer to the "no effect" end of the spectrum. Happy coincidence that (at least with movie depiction) we have colorful little pebbles!!

We can see the difference between an advantageous trait and a neutral trait demonstrated by the fur coats of wild cats vs house cats. Wild cats don’t have nearly as much fur variation within a species as house cats, because in the wild cats have a way higher need for good camouflage as it is crucial for hunting for prey and hiding from predators. House cats, however, coexisted with humans for centuries -- humans provide cats with shelters, food, and way more variability in types of environment (so a specific coat is not selected for) so unusual fur patterns could be passed on with way more success.

Colors are likely a byproduct of other physical traits, so let’s consider some physical traits eridians CAN perceive via sonar: 1) texture, and 2) silhouette.

Silhouette is straightforward enough: by how sound bounces off a surface, an Eridian can get the outline of another Eridian, down to the unique spiky protrusions of each finger. They can also get information via the texture of the surface: sound bounces off differently on different textures. This combined with silhouette can allow very fast person recognition!

The Eridian carapace is entirely inorganic, being composed of oxidized metals. If “Eridian” is all one species, all Eridians over Erid probably have very similar carapace makeup, especially Eridians living in close proximity, because they consume similar foods. But if they are different species of the same genus (think modern humans and neanderthal humans), it could allow more possibility for more carapace variation. (If you are interested in the speculative biology different species of Eridians, @lutheban has a great post on it under the tag #eridian biology !!)

Addressing the second part of the asker's question: can the colors be result of different minerals? While pure mineral/crystal Eridian designs are super cool to see, it unfortunately is not super biologically viable (unlikely to be evolved consistently). Eridian carapaces are comparable to shells and scales, being grown and shed as needed; it would take way too much energy to maintain pure mineral carapaces when a mixture of what is most available is an option. But at the end of the day, spec bio is spec bio: if Erid is strange enough, even the strangest traits can become necessary and be evolved.

Now that we know these factors, we can go into the main possible reason Eridians develop different colors: variation that comes along with the mechanisms that create the unique textures and sillouhoutes of each Eridian. Perhaps every Eridian breaks food down just a little differently, making the carapace a little different between each Eridian. Color, in this context, is similar to human fingerprints and palm etchings: due to it having no evolutionary effect, it’s allowed to randomize to no bounds -- to the point it’s pretty much impossible for two humans to have the same etchings!