Many others have explained .. and explained well.. the ways to use the cadmiums to best effect.
I thought at one time about eliminating cad red and cad yellow and cad orange from my inventory (they are not in my palette.. but available in tubes if and when needed),
but.... there are times when a small "pop" of cad red, cad yellow or cad orange..... used by themselves.... is just what is needed.
In a DVD I bought... Hazel Soan's "The Essence of Watercolor".. she gives the best demonstrations that I've ever seen for how and when to use opaque colors instead of the more often used transparent colors. I wish I had seen that DVD when I started learning watercolors a couple of years ago. I learned a lot from watching it recently. Here's the preview:
Also.. I'm watching a fascinating DVD by Barbara Nechis called "Watercolor from Within" where she uses opaque colors for very specific techniques and transparent or semi-transparent colors for other very specific techniques ... all in the same painting.
So I'm glad I have my cads because I'll need them to try her techniques.
Here's a link to the preview. This is an intriguing DVD... I can't wait to try it with my opaque cads