Correct. "green" is the experience you associate with electromagnetic radiation with a certain frequency. The electromagnetic waves exist outside of your mind, the experience does not. There is nothing "green" about the EM waves. It is only their interaction with the brain that gives rise to the experience of green.
Right. And it took a complex understanding of the physics of color to know that. And there are some interesting subtleties. Physically, mix of yellow and blue light is nothing like green light. But it looks just like green light to us. That's because of the quirks of how we perceive color. Our perception of colors results from a mix of the physics of color, the actual colors of the objects we look at, and the way our perceptual 'hardware' works. And we needed to understand the science of how light works and how our eyes work to sort that out.
The grass is still green only in the sense that because our brain would still function the same way, when we would see the same frequency of EM waves we would experience the same sensation.
When you say 'wipe everyone's brains', I can think of two different things you could mean:
1) We also wipe everyone's ability to empathize and our protective instinct we have for kids. In this case, we would not suddenly conjure up the idea that kids should not be tortured.
Of course. Poke out our eyes and we wouldn't know that grass has certain physical characteristics that cause it to look green.
2) We wipe everyone's memories and past opinions, but our evolution-given instinct to empathize and protect kids remains. In this case, we would think up the idea that kids should have the right to not be tortured.
Exactly. As long as we have eyes, we will see that the grass really is green. However, we may not quite know what's coming from the grass, what's coming from our eyes, and what's coming from the laws of physics. But the grass really is green -- it has real properties of the grass itself that make it look the color we call green.
For very good reasons, in ordinary cases, we simply say "the grass is green" as if this was a property purely inherent in the grass. We don't say "most grass looks the color we call green to people with ordinary color vision under typical lighting conditions". Why? Because when we say "is green", that's already what we mean -- that it looks green to people with ordinary vision under typical conditions. We should do the same things with rights, and we do. Most people just never realize they're doing that, just as they don't for colors, sounds, and so on.
We save these arguments for philosophy. When someone says "how do you know grass looks green to other people" and "how do you know colors aren't just in your head" and all that. But we all know these arguments are nonsense. It just takes us a bit of head scratching to explain why we were right all along, just as we knew for sure we were. You can try to call my perception of rights into question, but I know how to answer the objections. You cannot prove to me that I do not see what I know I see because I'm actually seeing it.