There are a number of reasons why canola in the vegetative stages may change colour. Purple coloring is often associated with certain nutrient deficiencies, but is actually just an indication that the plant is stressed. Here are some example of winter canola that has discoloured leaves and stems as a result of various stresses.
0:00 Intro
0:31 Hail causes purpling
1:14 Wirestem causes purpling and stunting
2:04 Nitrogen deficiency causing purpling and yellowing
3:40 Sulfur deficiency causes purpling and cupping (sometimes)
4:29 Herbicides cause discolouration and stunting