This is a classic pr0blem in philosophy, the problem of 'spectrum inversion'. Even if you see blue like I see red, and vice versa, it is very difficult to see how we could ever tell. I cannot see your experiences, and you would use the words 'blue' and 'red' the same way I do, since you were taught to say 'blue' when you saw blue objects and 'red' when you saw red objects, even if your experiences were different. It's interested that spectrum inversion is different from color-blindness. There we can tell, because color blind people can make fewer discriminations.
Read another response by Peter Lipton
The color of something is the color of the spectrum that isn't taken in by an object. However when I look at the color "green", do I see the same tint someone sees when they see "blue"? The identification of a color is what we've been told, and we've essentially been told what colors don't go good together. So how do we know that all of our eyes see the same thing? -Samantha B.