I is for: Iceblink
Iceblink is a noun for a yellowish glare on the underside of a cloud, signifying ice beyond the visible horizon. I know that seems pretty obscure, but bear with me. What I really want you to take from this is the occurrence of a very rare phenomenon: an iceblink rainbow.
An iceblink rainbow is basically a white-ish colored rainbow, caused in part by iceblink. Pretty cool, huh, a white rainbow? I think it is. Here is one of the few pictures I could find of one: