leaves with purple discoloration

how can I tell if it is a nutritional deficiency, or an excess of salts, which do not allow the plant to absorb nutrients well?

Potassium deficiency or super cold temps at night/— but your plant looks deficient in NPK all around.

guys, thank you. but if I go to see the potassium deficiency on the internet, no article talks about the purple color of the leaves, but rather yellowing.

Looks like they want some K (Potassium).

Yes your low on nutrients. More towards your Nitrogen and phosphorus ratios. Cause violet colors on edge of leafs. If was genetic won’t see until flower. Or also happen with very quick cold snaps. If pot/roots are on cold surface floor try lifting the pot off the ground(don’t use treated wood). These be the only two things I can things that I think is going wrong. If your nutrient is in check I would go after the cold snap issues.

are you saying it's a fertilizer deficit?

Looks like you in need of feed. Heavy npk ratio.