If they want to add weight all over or just in certain areas, warm colors such as red, oranges, yellows, plus white will do the trick. Understanding line is also important. Lines, whether they are stripes in the print, seams or zippers, can add width or length to the garment. Vertical lines add dignity and make the body look longer. Horizontal lines are relaxing and add width.
Diagonal lines add motion and action.
Proper use of the diagonal line can cover a lot of figure flaws by creating exciting optical illusions. Lines can make the bosom appear more seductive and fuller, the waist smaller, a torso longer, flatter a tummy, widen shoulders, and slim hips, when needed. The size of the print is very important. Large prints will add weight. This may be desirable when trying to create an illusion of a fuller bosom or hips.