I think it's really nice when people just happen to wear things that 'go' with the wedding. I mean, aside from people wearing wedding-dress-like dresses, it looks great. A black tea-length dress is fine for the wedding described in the OP.
Never heard of anyone complaining, "All the groomsmen wore black suits and blue bowties, and then the groom's colleague wore a black suit with a blue tie, too! I bet he was trying to look like a part of the wedding party!"
At our wedding, we didn't have a wedding party, but we did encourage people, if they asked, to dress along the same lines as our vaguely outlined colors (basically, purple, particularly lavender, with greens and neutrals) if they wanted, and dressy/churchy summer clothes.
Given that starting point and the fact that basically only moms, BFFs, and sibs asked at all, everyone looked GREAT in our family and friends portraits. No glaring reds, ball gowns, or T-shirts. Lots of purples, blues, peaches and greens, ties on guys and mid-calf to cocktail-length dresses on the women.