Are you talking about coat and tie, here, or casual wear? For coat and tie, your shirt should pretty much always be the lightest and least-saturated thing you have on.
For casual wear, I guess if you wanted to wear a navy polo shirt or casual shirt with light blue chinos, you could. I'd be more likely to reach for khakis. I wear a light blue oxford shirt with dark blue jeans all the time. I have one dark gray shirt, but I don't really wear gray pants casually.
To be honest, I don't think about it in terms of where I'm wearing a light color and where I'm wearing a dark color. I just make sure I have enough contrast between the shirt and the trousers that I don't look like I'm in a jumpsuit.
I don't think I'd wear a gray suit with a gray shirt. I don't think I'd wear a gray shirt with coat and tie at all. Gray suit, white shirt, gray tie? If that's how you want to dress. Forum member patrickBooth does that, and it looks good.
There's the idea of the Italian background -- navy tie, light blue shirt. There're many people who'd wear that with a navy suit. I might, but then again I might not. I wear sportcoats more often than suits, and you need more color variation to make that work.