We see lots of these at our place but this one is the darkest I've ever seen. It was almost black all over apart from the stripes on either side of the vertebral ridge. I can't work whether it's the much greyer individual I usually see nearby, but darkened by sun exposure, or a visitor. The grey animal does often appear to darken after basking.
It's one of the darkest specimens I've seen as well. It's probably a male, which can be quite dark, especially on the throat. As you mention, they also darken when trying to warm up and perhaps behaviourally as well.