Bean

I seem to recollect from somewhere, that Marius required a few things made en masse for his Head Count armies.
Amongst those the sagum, a particular cape made in Liguria which served to protect the legionaire from cold&rain etc.

I seem to recall this cape was usually red (why escapes me), which might be why we associate romans with red


Do keep in mind that Mars = a red planet. Red was associated with ares and mars and later with mars bellona.
Which might be why they painted the cape red in the first place, dunno.



As for the rest of the topic, uniformisation was present to a certain level.
Especially after Marius, when Quintus Servilius Caepio junior erected massive towns dedicated to make army equipment.
As for the shields, they were probably created uniform (blank) and then painted by the soldiers.
Painting upon production would be both costly and time consuming.