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An osciliateing fan fattens up the stalks, stems, whatever you want to call them becuase it forces the plant to keep growing towards the light wile being slightly pushed over so it has to keep getting back up, think of it that way.
If you had a fan blowing straight on them the whole time though youd end up with plants that are just blown over in the direction of the air.
Its not fully necessary to have the fan in there, if the plants are staying indoors they will be perfectly fine the way they are, the branches might be alittle weak when it comes to holding up the budmass but that can be easily fixed with string. If your planning on putting them outside though a fan is pretty much a must.
As for the lavender tint on the underside of the leaves, its either genetics or just becuase their still young or the temps are getting to low at some point.