User blog comment:ChishioKunrin/Red Myrtenaster - Before Dust was color coded/@comment-24.104.77.242-20151118210324/@comment-4010415-20151119014026

All I know is, they planned out the course of RWBY's plot before they began writing the script for the very first episode. They probably just hadn't yet worked out the finer details of Dust.

Also, it's more like Dust is mapped to various elements, rather than just natural phenomena, unless that's what you meant.