Yea that's it's spacial color coordinate
Yea the red x is like the dominant color of the blended light.
Simplist use is that 2 lights with the x in the same place should look the same.
And the predominant tone is, in this case blue.
This gives a bit more depth :
Coordinate system - x, y Cartesian coordinate system.x – represents the relative magnitude in relation to red; y – represents the relative magnitude associated with green; z – represents the relative magnitude associated with blue. and z=1-(x+y)
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By itself it didn't really tell you how " white" it is.. My point was just to try to define it against other lights .