There are pre-cut color packs that graduate in colors of the rainbow. There are hand-dies that graduate in color
(Laura Wasilowski )
Hold on to your pocketbook when shopping hand-dyes
There are fabrics called ombres that graduate, commercial, so less expensive that hand-dyes.
http://www.nancysnotions.com/category/fabric/ombre+fabrics.do
There are Classroom lessons in color by Laura Nownes here on TQS
Show 709 may have some hints on choosing fabrics as well.
[url]learn/classrooms/[/url] is about the way Jinny Beyer chooses colors.
We all have our individual preferences. I'd just say, remember value so that your piecing stands out against the background.
Line your colors out on your table. If one screams "out of place", don't use it. If you put something in your quilt that you don't like, you can always use that seam ripper to do it over.
Hope this helps a bit!