Currently, purple belts (53 items/sec) are the equivalent of a blue and yellow belt.
blue and yellow make green.

The green belt (66 items/sec) is equivalent to a blue belt and a red belt.
.....blue and red make purple. lol.

literally just a few name changes in the graphics folder would fix :3

it used to be that way around, but I changed them around so that green is the top end tier rather recently, because I'm working on changing the colours of everything to go grey/yellow/red/blue/purple/green. (When I'm actively modding)
Most of the top end things were already green, hence the reason I changed the belts, rather than all the other entities.