What is the reason behind the colors used for different flight categories? In Skyvector, Foreflight and Aviation Weather Center the following colors are used for flight conditions:


LIFR = Magenta 
IFR  = Red 
MVFR = Blue 
VFR = Green


In most contexts, red is used as the color for the most caution/hazard, but LIFR has lower weather conditions than IFR. Why use pink for LIFR?  

Somebody may answer: because LIFR is a subcategory of IFR (or MVFR is a subcategory of VFR).  Are there any other IFR subcategories (other than LIFR) to justify branching off that category? It seems like a creating a subcategory branch should have at least two categories.
VFR and IFR existed first. LIFR and MVFR were added later. I don't have a source on why magenta was specifically chosen, but red couldn't be used for LIFR because it was already used for IFR and in aviation we like to avoid obsolescence whenever possible.

I haven't been able to dig up exact timeframes for the implementation of IFR (since VFR was the default.) Although the terms VFR and IFR appear in the history of the FAA in the 50's, some references such as FAA's historical perspective cite the piedmont mid-air of 1967 as the source of the modern culture surrounding ATC including IFR rule enforcement.

For more reading on how flight rules came to exist in the US, I recommend an article on gizmodo.