I think that is just the "suntan" that leaves show when they are near the upper limit of the light they can tolerate. As the sun shifts, make sure that it's not getting too hot from direct light that it might be suddenly getting.
That sounds like low enough light... Unless that changes more, it is probably OK - go out there at midday and make sure that the light isn't sneaking around that shading some place - like from an open vent.