If I recall correctly:
Blue: Left
Yellow: Right
Orange: Both
I can't play surround audio without downmixing, so I don't know how it looks with that.
A key in the upper left corner would certainly be nice.
A key would be especially helpful because the analyzer eventually (with five or more channels) reuses colors, or uses colors that are so similar that I can't distinguish them. Additionally, the 5.1 and 7.1 standards put the channels in different orders, so the color that the analyzer assigns for the Surround Left channel in 5.1 would probably be assigned to the Rear Left channel with 7.1
I can't verify the assignments for sure right now because I'm not at home, but my recollection is that (in 5.1 mixing anyway) the Left and Right are as described above, the Sub Channel is pale blue, and Surround Left and Surround Right are Orange and Green respectively. That last can be confusing because whichever surround channel is orange looks to me like the same color as the "sum" line (i.e. the orange line that sums all channels). I do not recall what the Center channel's color is because I do not use it.
