I think the most you can say is that you and I would agree to call our shared experience of an object reflecting 700nm light "red". We would remember this and describe another light source which produced a similar sensation as "red". That's all.Well it seems to be a very interesting topic.
To me, for example, I will first assume that the 'truth' (that red is red) is not the same as our knowledge of the 'truth' (that red is red but not blue). I'm not sure why we should differentiate red from blue, and why should I feel the same red as you are. But I'm sure that we both agree "700nm represents something red",
I hope this is not much too 'philosophical'.
We "differentiate" red from blue because evolution has made it a favourable characteristic. 'Colour blind' humans would have not been as successful, because there would be situations in which their failure to discriminate would affect their survival. Likewise, development of sensors with more discrimination (perhaps using four analysis curves) would have been more energetically costly for little advantage. This is 'why' tristimulus colour vision is what we have. The 'accuracy' /agreement between individuals is also something which relates to cost - benefit in the 'design'. (you know what I mean - I don't mean that old fundamentalist rubbish)
Individuals can have pretty wild differences of opinion about colour matching under some circumstances and for some colours. Mostly it doesn't matter so we aren't likely to evolve to improve it. We can discriminate much much better than we can describe. 256 levels for each channel are needed for acceptable RGB representation of colours on a display (to avoid visible contouring on large nearly-plain areas of colour), which is where the 'Millions of Colours' bit comes from. We can't actually remember more than a few dozen / hundreds of colours well enough to be able to carry a match in our heads which another person might agree with.
My view is much more pragmatic than those who say "red is red" and that there's something fundamental about it. It's all 'learned' by our brains, according to what sensors we happen to been been born with and what our peers tell us.