Well, now, since you asked, I have to start thinking about it for the first time...
1. I guess it should be a light, or even a pastel color, with a brightness similar to the yellow that's already in use.
2. My own preference would be for a light blue; but maybe a pinkish color, or even a pale green, would make the chart appear less busy than a blue would.
Just my 2 cents' worth.
Well, I went with dark blue. The reason being that I had already picked a light blue for part-time channels. Anything and everything can be changed however. We'll see how it goes.