“What about red-orange?”, you say.   “Red-orange light would also simultaneously
 stimulate the Red and Green receptors.  See?”
Aaahhh…..true. But look!  The relative lengths of the arrows are different.
The green arrow, i.e. G-receptor, is much less stimulated than the R -receptor.
So, red-orange light is seen as different from yellow light due to
the ratio of R and G response.