The use of filters can help the photographer emphasize these differences. A UV filter is great to cut through haze and improve clarity. A polarizing filter is perhaps even better to isolate the different areas of the clouds and highlight their features. When doing black and white photography, a red filter is a plus to use to make the clouds really stand out and appear bolder.
A good time to photograph cumulus clouds is both before and after a storm. In California where I live, high clouds will normally precede an approaching storm front followed by more and more cumulus clouds as the cloud cover drops lower and the clouds thicken. At anytime during this process great opportunities for photographs exist. Here in California and other desert areas during the summer monsoonal rain season, thunderheads will often begin to build up over the mountain areas. These gigantic cloud formations stretching thousands of feet into the air are particularly beautiful to photograph as the sunlight plays upon their different features.
Always use a study tripod so that there is no camera movement. The evening is a great time to photograph thunderheads as the light at this time will give them a beautiful reddish glow. Also, scattered clouds at just the right particular height will take on some beautiful warm red to fuchsia colors. I love to photograph clouds that are categorized as “linear ventricular”. These clouds are long horizontally and have the shape of a “flying saucer”. They too are often found during the monsoonal season hanging over mountainous regions.