In a fleeting moment, the crew saw what they described as something "quite large and blue and yellow" pass underneath. The nature of the sighting is captured quite adequately in the transcript of the subsequent conversation with air traffic control at the airport.
"…we seemed to only miss it by a couple of hundred feet it went directly beneath us … wherever we were when we called it in it was within about ten seconds… couldn’t tell what direction it was going but it went right underneath us."
The object or aircraft was not tracked by radar either by the plane or at the airport - and the report itself thus concluded that it was unlikely to have been small light aircraft (such as a glider or paraglider) or one of the usual suspects such as a weather balloon.
It is tempting to point out that the test livery of the F-35B Lightning II - soon to enter service with the RAF includes a blue and yellow tail fin, but really there is nothing substantial in the report to justify any kind of identification along those lines. It might also be worth noting that bright colours like this are also typical of commercial dirigibles - notably the famous Goodyear blimp which is, of course, bedecked in the blue and yellow of the eponymous firm.
I've just noticed that Doubtful News have also suggested the Goodyear blimp as a candidate for this particular UFO. Goodyear have recently announced their decision decided to stop flying airships in the UK but throughout 2012 the blimp was still performing its usual duties up and down the country and was, by coincidence, responsible for a brief rash of 'UFO' reports during the Olympic opening ceremony earlier in the year.
Against this suggestion must be weighed the fact that neither the aircraft's own radar, nor facilities at Glasgow airport picked up any sign of anything in the airspace at that time (although nearby Prestwick airport reported a short-lived 'unidentified track signal' - in the area). As Airprox confidently ruled out weather balloons, man-carrying balloons and microlights on the basis that they would have have been 'radar significant' it seems unlikely that an object the size of the blimp would have remained invisible to their equipment.
[UPDATE: 15-5-13] I asked Goodyear directly whether the blimp was flying in the area at that time and they confirmed it wasn't. The mystery (as they said) continues.
It has been mentioned that there is a branch of Kwik Fit in the locale, who also have a blue/yellow colour scheme. The motor trade is a relatively prodigious user of promotional blimps - and Kwik Fit are listed as a customer of one of the UK's blimp companies. That is an avenue worth exploring, but it must be noted that such blimps are relatively small and tethered much closer to the ground than suggested in this case.