Is blending of colour lights a linear system?
Your mistake is that you are describing subtractive color mixing, not applicable in this scenario.

If you shed two lights onto a diffuse surface that does not absorb any light, and collect all the light diffusely reflected, the sum will be an additive mixture of the two. Incoherent (non-laser) light can be thought of as noise that has some electromagnetic spectral power distribution. From the signal processing point of view, long-time average power spectra of independent noise signals mix additively when they are summed