Purple stems are normal with some genetics, and can also be a sign of phosphorus deficiency but to my knowledge aren't related to magnesium deficiency.
Epsom salt should be applied at a rate of 1/4 teaspoon per gallon of water.
It probably is about time to begin general ferts as well. A 1/4 strength dose of Peters 20-20-20 would go well with the epsom salt.
