Cold? Yes.
Too cold?...
Lots of lizards turn dark when cold, frightened or stressed, and AFAIK, most cold-blooded animals slow down when cold because their
metabolisms slow down when cold.
https://en.wikipedia.org/wiki/Ectotherm
The opposite happens when they get warm—they lighten in color and move more quickly (sometimes to get away from
too much heat).
These are natural occurrences and perfectly normal—nothing to be concerned about if within the species' temperature range and the animal is otherwise healthy.
Since
Chamaeleo chamaeleon has been living on Malta successfully for ~150 years...
Just out of curiosity, do you recall any die-offs of these chameleons after particularly cold (or hot) weather? I couldn't find anything, so again, it sounds like they've adapted quite well.
You've probably seen this thread from 2013; I just came across it:
I also came across this article:
which says,
in part:
I'm not suggesting or implying anything—just passing along some things I found.