It is okay to wear a different colored shirt (than white) unless you are a member of the wedding ceremony and all are wearing white shirts. This may subdue your presence in the crowd without distracting, by being unique to the rest of the attendee's attire. I am thinking of group photos where only those closest to the camera show the clothes covering their legs.

I fall back on the old rhyme, "April showers bring May flowers." I was married on May 1st and believe in promoting color for the season and event. My best wishes for your friend.