you need to define the environment to get a better diagnosis. Temps (day -night), humidity (same), strain, floor (under the tent - is it cement, etc. - where is tent located), what you are feeding them and lastly the light (spectrum / distance, strength)
It will not hurt to spray with the epson salt mixture - 1 tsd per gal water, only at lights off or first thing lights on...If it is a mag issue, this will help in just a cpl days
Some strains, when the temps swing or are colder, will result in red / purple stems, leaves
Light too close can do this
Plants overall look very nice, do not overthink this...purple does not hurt
look at my GDP - a natural strain with heavy purple colors