The following is an excerpt from the Canola Encyclopedia:
The majority of the canola green seed problems in Western Canada are usually the result of frost. Even a light frost can fix the green colour by damaging the enzymes that clear the chlorophyll in higher moisture seed, preventing additional clearing regardless of how favourable weather conditions may become. A killing frost also causes rapid dehydration of seed and plant tissue, which may be just as important for reducing the ability of these enzymes to function properly. The seed enzymes that clear chlorophyll work rapidly at high temperatures but slowly at low temperatures.
Freezing temperatures have much less effect on seed chlorophyll content at lower seed moisture levels. Seed at less than 20% moisture should typically be safe from frost damage. Swathing one to three days prior to freezing temperatures may reduce seed chlorophyll levels by allowing for a more rapid seed dry down when compared to standing crops at the same stage of maturity. The time required to dry down the swathed crop to a safe moisture level is always dependent on the weather conditions experienced following swathing.