The Color of Magic

[Cleretic]

In all honesty, I think you're just observing a general tendency of the developers to use color as a major element of distinguishing between similar aesthetics. There's a lot of passingly similar elements to Allag and Garlemald, so a big way you depict them differently is through color; Garlemald's surfaces are much darker, and its lights mostly lean towards red circles and conventional lights, while Allag is brighter and is typically lit through lines that are usually in yellows and blues, and occasionally greens; usually by the time reds enter the color palette, it's pretty deep into an Allagan thing to the point where there won't be confusion. This is especially noticeable with the high-tech civilizations thanks to that coloration usually affecting the lighting, but you can see it in other places, too.

Solution Nine is probably very purple because that is a primary way to differentiate it from Allag; if the City of Gold were actually predominantly gold it would be too easily mistaken for some Allagan areas. (And probably also parts of Ronka.) It's not the only instance of purple tech in the game, but it's visibly extremely different from the other main instance: Mhach.