The relationship between the frequency and the wavelength of light is   C = l f.  Where C is the speed of light in meters per second, l is the wavelength of the color of the light in meters, and f is the frequency of that color of light in Hertz.

Thus the equation for l is: l = C / f   Likewise the equation for f is: f = C / l .

The l of the first subharmonic of the Upper Sideband of the mixture is lm = C / (f1 + f2 ) / 2
Where f1 is the first frequency of the mixture, and f2 is the second frequency of the mixture.

lm = C / (C / lR + C / lG ) / 2 by substitution of the frequency equations for Red and Green light.
lm = 2C / (C / lR + C / lG ) by simplification.
lm = 2C / C (1 / lR + 1 / lG ) by factoring C out of the bracketed term.
lm = 2 / (1 / lR + 1 / lG ) by cancelling out the C's in the numerator and denominator.
lm = 2 / ( 1 / 680e-9 + 1 / 515e-9 ) = 2 / (1,470,588.235 + 1,941,747.573)
lm = 2 / 3,412,332.808 = 586.109e-9 meters or 586nm. A nice bright Yellow.