That is a good question.
For matching fabrics, I think it helps to look at them on the same website (same vendor). If the vendor doesn't have that exact fabric, maybe they've got something from the same line with the same color scheme. So far I'm running @ 9 for 10 in gauging color matches on screen.
For backings, I usually don't try to match colors exactly. I want something that looks nice with the front, of course, and that isn't the same color as any binding I'm using. Swirly/batik-y things work well. I did just back a quilt with an Aunt Grace (or similar) red and white print -- from thousandsofbolts, in fact
. The red tone didn't exactly match the red on the quilt front, but they still looked good together.