Color scheme for websites...experts?

[JGjg]

how do expert graphic desingers pick the best basic colors for their sites?
I think this makes a huge difference, but it isn't easy.

I learned a little bit of theory on this, about using a color wheel and picking complimentary colors, but there are some gaps in my knowledge and it doesn't seem to work.

does anyone know of a really good guide on this or a special tool online?

thanks