I guess having 20 years of artistic experiance is what helps me choose colors when I'm designing a quilt.
You shouldn't have all dark or all light colors. Well, you could, but it'd be more pleasing if you had both dark and light colors in your design to contrast each other.
Using neutral colors (black, white, grey) can help spruce up any color scheme and I'm finding using them in a quilt can make your quilt design stand out a little more.
You can't go wrong with different values of one color.
It helps if you sketch out your design and color it in so you can see if the color scheme you have in mind would actually work. Also, know what you like. For example, I hate pastels so I'm not going to want to make a quilt that looks like the inside of an Easter basket. But I love bold and dark colors so I'd probably quilt something that should be up on display at a Hot Topic store. If you don't like a color or how two colors look together you certainly don't have to quilt with them. Go with what you like.