i was poking around on the internet just now & found this article from Kent Kobayashi from University of Hawaii at Manoa. It says :
"In correct light, some orchid plants will produce a pigment that resemble plum colored freckles or suntan. This is an almost ideal situation for good blooming."
source:
ORCHID CARE--Kent Kobayashi
i think this is what has occurred, because the color change has happened only on the part of the leaves that are exposed to the sun. you can see in the 5th pic down that on the part of the bottom leaf covered by the top leaf, it has remained light green. like a sun tan almost.
anyhow, i will continue to monitor this phal for good measure.