A lot of online stores will send samples or sell you swatches.
It is really difficult to produce pictures that will render correctly on a wide variety of computer monitors.
If you're trying to match colors with fabrics you already have, some stores will do it for you if you send a sample of what you're trying to match to.
Most online stores, with the exception of a few giant ones, are small businesses, often owned by quilters, that will go a long way to make sure you're satisfied and have what you need to make your project successful.