Heres a good start for research:
http://solarcooking.wikia.com/wiki/Category:Solar_cooker_designs
Concentrated visible light will heat anything regardless of color, but the color black (or any very dark color) will convert invisible UV energy into heat. So if you use a dark color on the focal target, it will get hotter and get hot faster.
If you've ever gotten sunburn on a cloudy day that had a cool breeze, that was the invisible UV rays that can penetrate clouds.
One of the projects is a pyrex (high-temp) clear glass bowl that has a black cast iron pot inside. Both the pot and bowl have lids.
If you turn on your cars heater full blast on a cold night, but drive with the windows open, it has the same effect of using an open collector and an open pot.
If a poor third-world-village only has an open concentrator and an open pot, its understandable, but if you have it available...
A suprisingly small concentrator will produce very hot and fast cooking if it has a glass cover thats sealed, and the focal point is a black sealed kettle inside a clear glass sealed bowl.
If you're cooking dry beans at a family bar-be-cue as a demonstration, a pressure-cooker will keep the water from boiling away and will cook faster. This is because when water boils away, it carries off heat in a vapor, and the pot never gets above ~220F. Under pressure, the pot can get above 300F. Just some thoughts...