i've been buying fabric from fabric.com for over 10 years and have never been disappointed- there are lots of other shops out there too- & i purchase from a few others at times- i believe most really work hard to make sure they have good pictures with true colors- but individual computer monitors do vary- some shops (like fabric.com) do offer swatches so you can order a number of swatches & make sure you like what you want before commiting to yardage-
i am (picky) about shops- and shipping charges- i generally shop at the ones that offer free shipping at a reasonable purchase amount --like at $35-$50 spent....if shipping is going to cost as much as a yard of fabric or more i tend to look elsewhere.