I'd water the plant with the Epsom salts added to the regular water you give it for a week or so. (about a tsp. to a quart of water) You should see improvement in the color of the leaves in 2 weeks or less, if the plant is still growing.
My orchids responded in about a week, but our weather is warmer than yours so they may have been metabolizing faster. But magnesium is very quick to be absorbed and used by most plants.
It was thought for years that reddening of foliage was a response to cold weather, but (at least in orchids, anyway) it's now widely accepted as a symptom of magnesium deficiency. As the weather cools, and light levels decrease, people slow down or stop fertilizing, so the little bit of magnesium most plants were getting is reduced or stopped altogether. The plants develop reddish tints to the leaves as they run out of magnesium, make less chlorophyll and can photosynthesize less and less.
It's best to continue with a little bit of magnesium in all your water through the winter when you're growing tropicals that (in nature) don't go dormant. You can let them slow down by reducing fertilizer, but keep them healthy with enough of stuff they are still using.
