My team has passed inspection with a little bit of the other color showing. Usually, the inspectors have no problem with it, even at championships, since it's obvious which color the bumpers are supposed to be. From a spectator's view, it's hardly noticeable, if at all noticeable.
But every inspector may be a little different, and I would advise having a plan in case they rule your bumpers to be illegal. Have some extra fabric ready just for good measure.
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R27
Each ROBOT must be able to display red or blue BUMPERS to match their ALLIANCE color, as assigned in the MATCH schedule distributed at the event (reference Section 5.1.1: MATCH Schedules). Markings visible when installed on the ROBOT, other than those explicitly required per R28, are prohibited.
R28
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 (for the purposes of R28 a round or circular FRAME PERIMETER has no corners), and
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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As a prospective inspector for future FRC events, this is what I would say:
It will be obvious to inspectors that your bumpers are either in the red or blue configuration, when in accordance to the match schedule. Arguing that a little bit of corner showing does not meet R27 criteria could be "lawyering" the rules a bit, in my opinion. That small amount showing on the corner is hardly enough to say that the bumper no longer qualifies as a red or a blue bumper. It is very clear which color you intend the bumper to be. An illegal case would be making a bumper half red and half blue on one side, which would definitely go against R27.
R28 only refers to corners when numbers wrap around them. I would not consider a slightly exposed corner a "marking" as stated in R27. I would say that R27 most likely refers to "markings" as any type of symbol or graphic like numbers, letters, or logos that are applied by media like paint, embroidery, iron on, or dye. Any marking that is not a number that meets R28 criteria will not pass. R28, as it appears to me, exists to prevent teams from putting their team name, logo, or any other symbols on the bumper.
Here's our corner with a little of the other color showing. Our bumpers passed at all four competitions we went to in 2013, which includes St. Louis Championships. You can even see a sliver of red through the reversible bumpers on the robot behind us, and they're on the field, so they passed, too. A tiny bit of exposed fabric is kind of an inevitability with reversibles.
This isn't to guarantee that your bumpers will pass, though. Generally, we have some extra fabric on our corners, and we tuck and stuff it into the space between, so the other color doesn't show at all. We made sure to do that this year mainly for aesthetics. If your team has some extra time, try to add some fabric to the corners. It wouldn't hurt to do so.