It is not *strictly* a placeholder. There are examples of things that we are pretty sure exist, but just aren't like the normal matter that makes up your dinner table. And it isn't usually so much that you put a placeholder value in to make an equation work - that's just making crap up.
In some instances, the exotic matter has never directly observed the material, but *everything* we know and observe says it should exist (neutronium, degenerate matter). In others, it is the most reasonable physical interpretation that best describes what we see in the universe (Dark Matter and Dark Energy, f'rex). And in yet other instances, it is a result of asking, "Assume this effect occurs. What does that imply?" If the effect doesn't actually happen, then the material probably doesn't exist. This is the wormhole-exotic matter case. This last is the more speculative.
Well, in most cases, you are positing the existence of a material to fit a very particular problem. In such cases, you are fairly restricted in terms of what properties the stuff can have and still be consistent with what we do know. Dark Matter, for example - if it interacts by means other than gravity and perhaps the weak nuclear force, it won't fit what we observe in the universe.
If you want ot create complete fiction, and posit a material with arbitrary properties, that's fine, but this solves no problems.
Well, individual particles do not have "color" in the visual sense, nor texture. These are properties of matter in bulk, often dependent on how the material comes to be - carbon can be opaque grey graphite powder, or clear solid diamond, for example.