I choose a focus fabric and pull from my stash before I buy more fabric. I love selvages, but honestly don't look at them when I'm choosing fabrics. I tend to have a lot of movement in my quilts, whether that's from the prints in the fabrics, the project pattern, or a combination of both. I don't think there's an exact science to it.
Maybe it would be easier to do some small combinations from your stash to get a feel for the playfulness of choosing before committing to a whole project. You could always do a sample from fat eighths or fat quarters before spending the money on yardage. After you do a block, let it sit for a couple of days as others have suggested to see if you'd like to change anything. Many quilters find their projects develop over time, that they are tweaked from the original plan.
We all have projects we loved at one point but didn't quite turn out the way we'd expected or we outgrew. Choose fabrics you love and have fun. If you are becoming frustrated, look up the color wheel online or head to the paint department at one of the home improvement stores. You can look at solids together without the distraction of prints.
Also, I assign a color based on the overall look. Sometimes people get lost in the various colors within the print. If it is overwhelmingly yellow, I call it yellow despite the fact that it also has blue, orange, and black on it. I may consider some of the other colors from the focus fabric, but only if I want to accentuate them.