Well, I think you are referring to a classic trench coat. I personally think that a modern trench coat can be single breasted. I know a lighter color makes me look bigger but somebody said that a small guy (shorter than 6') shouldn't wear a double breasted trench coat (especially a light color double breasted trench coat). So, I'm wondering that should I wear a light color trench coat (single breasted) since I'm only 5'8"
What you are referring to is not a trench coat. You can certainly have single breasted raincoats, and they look perfectly fine, but they are not trenchcoats. Words have meanings, and trenchcoat is a specific type of raincoat, not just a word used to describe any classic raincoat you want. Basically, double breasted and belted are the key elements to a trenchcoat- some would also say it has to be mid length, but makers have been playing around with the length from day one, so I wouldn't include that.
As for color, your size doesn't matter, whoever told you that you shouldn't wear a real trenchcoat was wrong. Wear what you like, just make sure it's sized appropriately. I'm 5'6" and own both a classic tan trenchcoat and a black Mackintosh (single breasted raincoat with a covered fly, probably what you're looking at). I find the trench gets more wear when I'm dressed fairly formally and the Mac more wear when I'm in business casual. YMMV. Tan is certainly a perfectly acceptable color for a smaller guy to wear, as is navy. Pick whichever you like.