Shields?

[Tellos Athenaios]

Red is usually the cheapest dye around. Purple, rich blue or green would have been more expensive, hence purple being the more heirachy colour.

Rich, lasting, color of any kind would've been expensive: the cheap dyes usually fade in sunlight...
Incidentally, rich & lasting red would've been very expensive, similarly expensive as Tyrian purper.

However, weld (yellow), woad (blue), madder (red), lichen (yellow/brown -> red/purple) are all fairly cheap.