Your brain see it yellow, but it's not, it remains a mixture of both wavelengh.
Mixture : both wavelengh exists at the same time each correspond to specific photons with different energy. our eye see the "mixture" as yellow, but it's purely arbitrary. We could as well see both color at the same time instead of a new one.
Reality appears according to the tools you use to look at it. Change the tool and you change what you see.