Some phals have a dark color on the underside of the leaves, sometimes even a dark maroon color. I think yours is probably fine. The roots look very dehydrated though. You may want to consider misting the aerial roots a few times a week. It also looks very dark green. If you could give it a little more light that might help also.
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those roots have heen that way for a while. i noticed thos when i did some "investigation" while repoting. the root sysem was waaaaay worst than it is now, but it still has me worried about the health of it. i just ordered some seaweed extract so hopefully it will help tremendously.