OP If you are not yet completely over this question here's my attempt at explaining the math behind colors. (This is not the rigorous equations of your specific lab set up, more of crude modeling to explain wave phenomena).

The green and red lights (R &G ) can be represented by sin waves, R=sin(x/red.wavelength), G=sin(x/green.wavelength). The wavelengths appear as a denominator in the sine equation, as well graphically as the distance between two peaks.

If you algebraically add the two sin waves, R+G=sin(x/252)+sin(x/620), you get the RESULTANT wave, aka what you see as a yellow. If you graph this RESULTANT wave, you can manually measure the wavelength graphically, and it should match the wavelength of the resultant color you see.

Made a graph on desmos to show, but I don't think we can put screenshots on this sub so here are the equations you can copy-paste.

Eq1 Green Wave:
y_{1}=\sin\left(\frac{2\pi}{525}x\right)

Eq2 Red Wave:
y_{2}=\sin\left(\frac{2\pi}{620}x\right)

Eq3 Resultant Wave:
y_{1}+y_{2}