Most times when I need a small inductor, I try to find the necessary core in my junk box. Depending on what I'm trying to build (RF, power, ...), a ferrite core or an iron powder core might be the better choice.
For completeness, ...
Magnetically soft ferrites, as used for today's RF or power applications, are sintered particles of, mostly, MnZn or NiZn compositions. (Wikipedia section about how they're made)
Iron powder cores are produced by gluing small iron particles together using epoxy resin or another polymeric glue. I've seen them pretty much exclusively as toroids. The main applications appear to be mains frequency input filters and PFC (step-up) stages at low to moderate frequencies, not so much SMPS transformers. One big advantage appears to be the fact that you can build toroids with an air gap distributed along the entire ring.
I am aware that many different types of both ferrite and iron powder exist (and it may be interesting to test them, too), and the differences do matter, but let's say I'm just hacking some proof-of-concept circuit and don't care about exact losses or saturation properties.
Still, I want to avoid really bad mistakes like using iron powder when I'm building an antenna balun. Some other time, an iron powder core may be just right and a ferrite core may be a bad idea.
Let's focus on simple toroids of different sizes, as a start, because this is what most iron powder cores look like.
Is there an easy and somewhat reliable test that tells you if your core is made of ferrite or iron powder?
Like winding ten or twenty turns of wire onto the core, carefully applying a rectangular voltage (low duty cycle, via a power MOSFET, using a freewheeling diode) to this inductor and looking at the point of saturation in the inductor current?
Or testing the inductor with a sine sweep up to some 10s of MHz in an appropriate circuit?
Also, can you sometimes tell just by visual inspection? For instance, are these color codes used by other manufacturers, too?