A picture is worth a hundred words. Without seeing it, I hesitate to comment.

Dahlia plants 'sport' to different colors all the time, but I haven't heard of a flame going purple. Sounds odd, and I wonder if another plant snuck in a branch in the flame plant's domain. Happens to me all the time, as I plant too close together, forcing me to follow the branch of a mystery bloom down to its main stalk and tag stuck in the ground so I can identify new varieties.

If you type in 'dahlia sport' in the above search bar, you'll find images and descriptions in previous posts on the subject.

Cheers!