Sometimes it is just "A Leap of Faith". I usually stay with colors I like, then choose several interesting fabrics and try them one at a time until I find the one that "jumps" while not offending the other fabrics but emphasizing the other fabrics. It is had to explain. But it gets easier with practice and if you shop in groups, ask others to assist. Of course, occasionally I just throw caution to the wind and pick. Start small -- a table runner/topper, build your confidence. You will probably surprise yourself.
My first experience was watching someone choosing two border colors and I was thinking "WHY?". But once they were added, it was like "WOW!"