It is quite possible that if inappropriate clothing is selected, the photographer may ask you to change your selection before the portrait sitting is made. Dark Clothing Tends to Slenderize Darker clothing helps to blend the bodies with the background, so that the faces are the most important part of the photograph. Dark colors definitely tend to slenderize the subject. Light colors seem to add weight to the bodies. The color of the clothing should always be toned down.
Bright colors attract attention away from the face.
Colors MUST be dark and not bright. For casual portraits it is equally important to wear long pants instead of shorts. When arms and legs are exposed to the camera there will be more flesh in the photograph in those areas than on the face, itself. This is a major distraction. A scarf loosely tied around the neck also helps to frame a face beautifully when a collarless blouse or sweater is worn.