	The problem of whether two people can experience the same color differently is a very complex one. I have considered the problem before, and read some about it, and draw the conclusion that it is entirely possible that Sheila sees green where Sam sees red, but there is no way for anyone to know. There are a number of seeming solutions, but I will detail why I don't think that any of them would really work. 
	The only way you can get information about another person's experiences is from what they say and do. If someone sees an apple, and says that it is red, you make the assumption that they are having the same experience of red as you are. The problem is that they learned the meaning of the word "red" by looking at the things that whoever taught them color words saw as red. If Sheila sees fire trucks, apples, and blood as being green, and is told while growing up "that fire truck is red", she simply learns to associate her experiences of green with the word "red". Where she sees green, she will say "red", in any behavioral test that can be devised, because for her that is what the word "red" means. The immediate thought about circumnavigating this problem is to find some objective description about what red things look like. An objective description of the color red is possible if you consider the meaning of the word "red" to be a certain wavelength of light, and you can use a spectrometer or something similar to say whether something is objectively red. The experience that you have when seeing red things, however, is not amenable to objective description. If you want to try and discover whether Sheila has the same experience of red as you do, you might say "it's like orange with the yellow taken out", and ask her if she agrees. This might work, if red is the only color she sees differently, but it will not work if her entire experience of colors is different. It is possible, for example, that her experience of every color is completely different from any color that anyone else has ever experienced, and that her red does look like her orange with her yellow taken out, because she also experiences yellow and orange differently. This also prevents the possibility of discovering her color experiences by saying "yellow is a really bright color, almost as bright as white", because in her own experience of color, it may be that white and yellow bear the same type of similarity that our experience of white and yellow bear to eachother, and that she has learned that the objects that we all call "yellow" and "white" are also considered to be "bright".
	A more sophisticated way to figure out Sheila's color experiences would depend on a mature neuroscience. Suppose that in the future, we can put a very tiny probe into every single one of a person's several billion brain cells, which will record exactly when the brain cell is firing and is not firing, without any harm to the individual. The results can be analyzed by a computer and correlations can be found between what color object the person is looking at, and what the person's brain state is at that moment. Suppose then, we find that when everyone looks at a red card, the neurons in some particular brain area fire at frequency R, and when they look at a green card, the same neurons fire at frequency G. Then we hook Sheila up, and show her a red card, and observe that her neurons are firing at frequency G. In a case like that, it is probably safe to say that she is experiencing red the way that most people experience green. But I will explain why I think that the opposite situation is not true: if Sheila's neurons fire at frequency R when she sees red cards, just like everyone else when they see red cards, it is not necessarily true that she has the same experience of red that everyone else does, even if the color processing areas of her brain are constructed in exactly the same way as everyone else's. If her experiences of colors were still different, however, it would lead to very bizarre consequences.
