Many things can cause purple foliage, but I'm betting it's a water-logged soil. You're seeing the magic of (plant) color expression resultant of one or more stress factors, and probably caused by the lack of certain nutrients. Purple coloration often results from breakdown of chloroplasts and the expression of anthocyanin and xanthophyll. Anthocyanin is a flavonoid, whose job it is to provide coloration for fruits & blooms & help protect foliage from UV radiation.
The most common cause of purple leaves (very common at this time of year) is a deficiency of phosphorus (P) because P is required to make ATP and ATP is needed to turn sugars to starch and to load sugar into phloem for transport. No P - no ATP - no move sugars/starches - anthocyanin (purple pigment) builds up - plant turns purple. The most frequent cause of a P deficiency is cold soils, which is why so many plants planted out too early turn purple.
A P deficiency is not the only suspect cause of purple leaves. Nearly any environmental condition that puts the brakes on growth and the accompanying use of sugars, but does not limit sugar production (photosynthesis) can cause anthocyanin buildup and purple leaves.
If only the outer edges of the leaves are purple, it may be a K or Mg deficiency. If the center of the leaves are also purple, it could be too much Ca in the soil or the result of too much water in the soil blocking uptake of P and Mg.
Do not apply a fertilizer with the middle number higher than either of the other two numbers (N & K) to container plants. There is never a need for that much P (relative to N) in container culture unless you're supplying only N in another form. Plants use about 6 times more N than P and the massive doses of P in 'bloom-booster fertilizers' cannot do any good.
Al
