Reds, purples, yellows, oranges, are all difficult colors to render perfectly. They are so saturated in color it's easy for them to blow out. Most especially anytime they are in direct sunlight. My suggestions are spot meter then go to manual mode, start with an exposure compensation of -1 and keep heading south until you get the accurate color. If you get to -2, use a diffuser and start over. By the way, when these same colors are in full shadow, getting accurate color is still an experimental process, partitcularly in the deeper tones. In short, in most cases do not rely on the camera's default settings when shooting these colors when their fully saturated.