I think the answer to this goes a bit deeper than what you or I might think is good.  Basic web design theory says you want to run a 3 color, color scheme. primary and accent main colors with a 3rd highlight color.  
With that in mind, then you ask what 3 colors..and the answer to that is not so simple. You can do a kick ton of research on the topic. Color theory and how it relates to the online experience is really kind of a science.  
One of the things that i do is go around and look at sites that are of the same topic as the one I am developing.  I specifically look at information based sites ( blogs etc ) I look at social pages and the color in the head banner.  I pay real close attention to commerce sites, but break his into 2 categories.  Small fry sites, and then big corporate sites. I also look at demographic information do I think the site will draw more woman, what colors do they like?  will I draw more men? what colors do they like?  the same goes for age break downs, what a 20 yr old likes is probably not what a 60 yr old likes.
Even after doing that I will develop a buyer profile and the people I want to target, and then seek out other sites that they may be visiting and buying from.  After all of that there usually is a pattern of color choice.  
Thats how I go about it anyways.