I expect that it is a cultivar thing - some cultivars will have some reddish-purplish colour on their pods while others may not. I also expect that the amount of sunlight and temperatures would affect how much and how deep the coloured areas become.
It might be interesting to check whether those cultivars that can develop reddish colour on the bases of their fans in the early spring can also develop reddish-purplish colour on their pods.
