ding ding ding! winner winner chicken dinner!
firstly, it was a cheap base compoud that was neutrally coloured - anywhere from grey, to light blue, to whitish yellow, depending on simple additional chemicals. it could be made into lots of other colours from there, and itmade those colours to be brighter than other base compounds.
and as you said, it kept those colours from fading and gave the paint a measure of weather resistance.
you're up alix!
A true lover of nature does not despair now that his mistress has turned a colder cheek.