from experiance of the yorkshireITALIA stickers. Multicolored stickers prove a problem because they are initially cut from a single piece of vinyl. then added to transfer paper.
Multicolor stickers would involve cutting several different pieces of material, peeling them from the backing paper, then reapplying to another sheet of backing, then transfer paper.
Multicolored stickers would be better if the were printed vinyl labels, but this process costs a lot more.
They would need to be special orders and cost about £5 as compared to £1 for the normal vinyl ones