I always get the question, "what should I wear?" It really depends on if you want a rural or urban location. First, let me explain your eye will naturally go towards the brightest part of an image. For example, if you are wearing a gray outfit and bright neon shoes, the first thing someone will look at are your shoes. Even if the subject eventually looks at the face they will always be drawn back to those shoes. Obviously, in a portrait, you don't want the dominance of the image to be the shoes. That is a very unusual example be helps clarify the point. A more common problem can be white. If you have on a white shirt and everything else in the image is dark, the eye will naturally be drawn towards the white shirt. Don't get me wrong white can work rural or urban. It really depends on the location.
A good rule of thumb is no white, black, bright colors or patterns. Simple earth tone colors work best. Rural locations are nice for children, families, teens and adults.
Urban locations especially neutral locations like concrete allow for more variety. Here you can wear black, colors and patterns as long as the background is simple and neutral colored. Urban locations work better for teens and adults. It really depends on the style of image you are after.
Below is an example of how the color of clothes affects an image. Look how the soft blue works with her eye color, the gray looks flat and the green overtakes the image. Bringing a few different outfits or shirts can be helpful to suit specific locations.