No- purple is not caused by nutes or the soil
Purple is the natural reaction of SOME strains of cannabis when they are approaching harvest-- it is the chemical reaction of the plant breaking-down the chlorophyll and producing /using the carbs in it's system to produce the yellows/reds/purples to protect it from the expected cooler weather... just like trees do in the fall.
It is a GOOD thing to have...
If a plant is 100% green, no yellowing etc at harvest- it is most likely harvested early- or grown in a 'warmer than optimum" environment.
YES- you need to flush the plants- especially if you used the time-released chemical MG soil... if it was the all organic-- you are fine.
Keep in mind-- some plants need more nutrients- and MG all organic soil is a great start, but some may need additional flowering nutes after week 4 or 5 of flower... but if that is MG time-released-- find different soil! you have zero control over the nutes, and it is too strong for many strains and you can't adjust it-- and you can not FLUSH it except by uprooting the plant, and setting it in a DWC bucket for a few days (changing the water daily)