Someone with more experience will come along soon but until then here I am! To answer your questions, it depends. What kind of average hood are you working with and what are you trying to get out of it?
Foil will not work & white paint is certainly better than nothing but they do make reflectors you can retro fit into the hood. Where the "depends" really comes into play is what you are trying to get out of the light. Typically, If you have a standard florescent hood light & want to grow plants, even a great reflector isn't going to cut it. With more details we can better assist you.
Also, you might get better answers in the Freshwater equipment, products & DIY board.