The signal the cones output that the brain gets is lossy, it cannot be restored to its original wavelength histogram, and in my book that qualifies as “combined”. If you’re trying to say that red+green light doesn’t turn into yellow wavelength light before hitting the cones, then obviously yes. Otherwise, it seems like you’re arguing against your earlier comment and not mine. Your example was yellow light vs red+green light. The physical signals that our brain receives are physically identical in those two cases, not just perceived as the same, therefore by your earlier definition of combine, the colors do combine before it’s perceived. Seeing red+green light as yellow is not a matter of the brain interpreting two different signals as being the same, the signals themselves are the same, before the brain ever gets the signals, as a byproduct of tristimulus response. We don’t need human perception to demonstrate this either, a digital RGB camera has the same property, that red+green are physically identical to yellow when you measure using RGB primaries, with no perception involved. If you want to tell red+green and yellow apart, you need either more primaries or different primaries, and perception isn’t part of the equation. Your original claim on this point that the brain is involved was incorrect. The retina is involved in the sense that the initial output of cones in response to seeing either red+green or yellow light is a signal we call yellow. The moment light was converted by cones into electrical signal, that signal has lost any information that could distinguish between red+green and yellow. And yes, all of this is supported by the link I provided on metamerism.