According to <R31>:
Quote:
Team numbers must be displayed on the BUMPERS and meet the following criteria:
-consist of numerals at least 4 in. high, at least ½ in. in stroke width, and be either white in color or outlined in white
-may not wrap around a corner of the FRAME PERIMETER
-be positioned around the ROBOT such that an observer walking around the perimeter of the ROBOT can unambiguously tell the Team’s number from any point of view.
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Those are the only requirements for bumper numbers, so as long as they are big enough and white/outlined in white, they're good.
Also, considering Two Pencil Designs, a business run by FIRST alumni serving the FIRST community, made bumpers with iron on numbers and posted them
here, that's also some good proof that it's ok.