Why is this Cuscuta compacta purple?

the Genus Cuscuta has over 200 species varying in color from yellow, to orange, and purple (sometimes but rarely can also be green).

The organism in the photo is not likely to be a subspecies of the usual dodders you find in your area, instead it is likely to be a different species entirely (while still belonging to the same genus).

The rope dodder (Cuscuta compacta) is very possibly the species you are looking at. However, I am also convinced that Cuscuta europea is a possible candidate, particularly due to this photo of the species that I got from the European Nature Information System (EUNIS) web site:

I must admit that I do not know nor have access to photos of all the different species of the Cuscuta genus to make a truly objective analysis of your image however I hope this information as a minimum leads to some new insight on the matter.