Hi mate,
Could be genetics (genotype) and environmental factors such as heat. 22 degrees is at the lower side of the temperature range so cool air may cause darker flowers. So the reason could be phenotypical expression (genotype + environment). But of course it could be some other issue. Does your plant look healthy in general? More pics maybe? If healthy, then I assume there is nothing to worry about. Maybe 5% of this strain produces dark flowers, and this is one of them.
Happy growing to you!
Cheers.