If this was 2012 with the intense sunlight and heat, the purpling would have just assumed to be sunscald.
This can still be the case in cooler conditions (as in late July of 2013), as the purpling is the build-up of pigments called anthocyanins. Typically this is a stress reponse of the canola to intense sunlight moreso than heat. If you are seeing a purpling on the top of the pod during ripening, and the underneath portion (not exposed to the sun) the regular green color, it is probably still sunscald.
One thing to note is that some varieties tend to show more of the purpling than others. This becomes important as you are assessing your crop for seed color change and swathing timing. S0me varieties can have green pods and ripe seeds and other yellow-purple pods and green seeds. Opening the pods starting at the bottom of the main stem (more ripe) to the top of the stem (less ripe) will give you the better idea of when the target 50-60% seed color change is achieved.
Prepared by: Anastasia Kubinec, MAFRI Oilseed Crop Specialist