1. Should she wear a dress? Not necessarily.
2. Can she wear a color matching the groomsmen? Absolutely!
Wearing a matching color, especially black, is very easy! She fits right in, but still looks feminine! In my research I also found the photo of a groomswoman wearing a dress matching the color of the tan suits the groomsmen wore. You can view that here. Love it!
As with every detail of your wedding, the look is up to you! My personal preference is the corsage to keep things cohesive with the groomsmen. But, I think either look is great! And sometimes (see above) the design of the dress doesn't work with a corsage.
4. What about color? Groomsmen often have some of the color of the wedding incorporated into their looks with the corsages, their ties, or cummerbund. Women can have the same splash of color with a sash.
5. Black and white dresses...
A few of the weddings I came across had black and white dresses, like a female-version of a suit.
I especially like this look. Not sure if it is a skirt and blouse or all one dress. I love that the top has a tie. It is still very feminine, but goes nicely with the men's attire.
6. Same color as bridesmaids, different dress.
In this instance the groomswoman wore the same color as the bridesmaids. She still is set apart by a few details. She is wearing a different cut of dress and she has a corsage like the groomsmen. I think this might not work as well if the bridesmaids were wearing the different cuts of dresses as well.