Howdy!
Reflection color is only available when working with the legacy MTL and what it does is it defines the strength & color of reflections. So 100% white for example means reflections at their full strength. 50% white (grey) means reflection intensity will be cut in half. Black means zero reflective properties (although technically you still have diffuse reflections but we typically don't call those reflections).
If you set the reflection color to full red for example then you'll end up with reflections that are red colored.
IOR on the other hand refers to the Index Of Refraction which defines how the reflections behave when you see the material head on versus when you look at its edges from more of a grazing angle. A perfect way to demo this is with a sphere (in reality and in CG). On a sphere you'll see that if you look at it straight in the middle part it'll look less reflective as it does on its "edges" (grazing angles, outer most parts of the sphere). That is basically the IOR effect. If you go with higher values (typically 2+) you'll notice how everything becomes more reflective all-around. If you go with lower values (anything above 1.0) then pretty much the only place you'll see reflections are towards those edges.
The Index Of Refraction also plays a part with refractive materials (as the name suggests). For refractive materials it defines how light "bends" as it travels through the material / substance.
You'll probably notice that the Physical Material doesn't give you the ability to tweak the reflection color when creating non-metal materials. That is because tuning the reflection strength and color via a single parameter like that isn't really... Physical. To control the intensity you typically want to either play with the IOR or with the roughness parameters (very high roughness value makes the reflections diffuse and they visually look less intense because of that). That would then be the physical way to go about things - and why the Physical Material is the go to material for Corona now as it is way harder to create a material that is not physical.
Hope that helps! :)
edit: Just to try and clarify things a bit more. Visually comparing the difference between reflection color and IOR would be that lowering the reflection color just lowers the reflective property for the material as a whole. IOR would lower it based on the Fresnel formula aka grazing angles could still be more reflective compared to viewing the thing head on at its center.
