Sam I could tell you how a 7th and a 6th sound different, but that is not going to help you when you have a different interval. Generally speaking, you listen as you play, and when something doesn't sound right, you check to see if you have the right notes - it is as simple as that. You are going along and then crunch - that's not right - better check.
Which is something synthesia doesn't do for you. It tells you visually that you have played the wrong note, when what you need is to recognize by ear when you have something wrong. You learn to do that yourself by listening as you play.
@Sam - the general premise is good and I agree with it. There are other factors, however. I've been interacting with my main teacher and friend for over a decade, and piano teaching is a frequent topic. What we may not think about or realize is that people's perceptions and abilities are different, and these abilities also change and grow. You may be able to hear that a note sounds wrong, while the other person doesn't have that ability. In teaching, being aware of this is critical. In my own growth, there are things I could not hear and now can, and there are still things I can't hear, even when they are pointed out to me. Or you may hear well in some contexts, but not others.
"when something doesn't sound right" presupposes the ability to hear that it doesn't sound right. Part of the teacher's job would also be to start waking up those senses by various means. Where you're your own teacher, (we all are to some degree) you also try to wake them up. You might use tools to do so.
As a personal example, one weakness were aspects of time. One thing I discovered is that I could hear when two notes were not in sync, but I could not hear which was before the other one. It was like an audial dyslexia. If I recorded my playing, and watched the bumps I could see what I couldn't hear (which sound came first). Watching while listening, one sense (seeing) helped develop another sense (hearing).
I can picture the Synthesia angle as either being a crutch, or becoming a tool for better reading (and maybe developing the ear?) if used judiciously in a good way. If it's still needed a year from now, then maybe not as good.
