"Emits light other than red" suggests that either another light bulb (reversing or turn signal for example) is coming on with the fog, as Sarah suggests, -or that you have a wrong colored bulb in there, like an Amber bulb.
Did you have the rear clusters re-coded in VAG-COM when you did it? -If so (or if not) are they CORRECT?
Also, you've told us what it failed for and what the wording is, but you haven't told us what's actually HAPPENING from your own observation. -Have you actually stood behind the car and made a note of what's actually happening when you engage the rear fog lights? -This is important information to all of us.
-Incidentally, IF their report is actually wrong, you can challenge the failure at another MOT test station, and have the cost of the original test refunded. -In such a case the MOT station which wrongly failed the vehicle initially gets a note. -Too many notes and they get a visit.
So I'm wondering if there really IS a problem. -Certainly, if the rear fog switch lights up either the reversing bulb or the turn signal bulb, then they are quite correct to fail the vehicle.
Also, the SPORTBACK 09 rear light clusters are not plug-and-play; since they have a completely different connector to the 08-and-earlier rear clusters, so you must have an adaptor harness in there somewhere.
Either a wrongly-wired adaptor harness OR an incorrectly-coded vehicle will cause the wrong bulbs to light up (since the tail lights of the A3 are definable in software).
Not sure if you have a sportback or a 3-door...?
So first we need to know what actually happens when you switch on the fog lights (I don't think you've told us that bit yet). -Then we mighe be better able to help you.
But there's no way a car can be failed "due to 2009 rear lights" unless they're wrongly wired or incorrectly coded, without the test station risking an appeal.
Keith