Thanks for reporting this problem.
It is caused by a misbehaving series aggregate that returns the point color as array - ["Red"] instead of "Red".
Your solution gets around this in a very interesting way. The String constructor flattens the array exactly like ["Red"].join(",") would.
This problem is solved as part of other fixes for the string aggregates and shouldn't manifest itself in the latest internal builds.
Apologies for the caused inconvenience.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI