While printing on yellow banner material sounds like a great way to save money (in yellow ink), it actually costs more! Therefore, we only print on white banner material.
When printing on yellow material (or any color material for that matter), white ink needs to be printed underneath the 4cp ink in order for the colors display correctly. White ink is very expensive and slow to print on digital banner equipment. Further, color banner material is more expensive than white material For that reason, it’s less expensive to simply print the yellow ink.