If you’re going to be a guest at a wedding, one of the cardinal rules for dressing is to never upstage the groom. One way to do that is to wear muted or unassuming colors. Navy, grey, or if you’re feeling adventurous, browns. For me, it’s a no-brainer.
Also, keep it simple. Chances are the groom may be sporting a tux, so as a guest, a regular suit with necktie – not a bow tie, which will most certainly call attention to itself – is a safe option. And since it’s spring, a silk knit tie is perfect.