As in the title. Let us distinguish a stack of red onions from a stack of yellow onions. Currently, both stacks share the same texture and are yellow(ish).
IIRC devs have said that variable materials on stockpiles would be too performance-intensive.
Separate stockpiles for red/yellow onion is probably a reasonable ask, and would only require Jorb to copy and recolor the existing onion pile model to make a Red Onion pile model.