The Diamond body shape has a full midsection starting from under the bust line which means your waist is undefined. The hips are wider than the bust and shoulders and the upper legs are usually bigger than the lower legs. If you have this body type you will usually gain weight around your midsection which includes your stomach, hips, buttocks and usually the upper legs sections.
Neutral colors are better for a person with this body type to wear because they make them appear to look slimmer. If they do wear bright colors it shouldn't attract attention to the undefined waistline.