Your face doesn't look darker to me. Accutane makes you photosensitive so it's possible you could have a bit of a tan on your face because it is exposed to the most sun. When I was on accutane I would get sun burned from barely an sun exposure at all.
I just realised that the picture is upside down My skin has been super sensitive while on Accutane. I think you might be right about the tanning. All of my friends at uni have definitely noticed a change in my face skin tone but my little brother was the last straw. I cant even use my laptop because my skin feels so sensative