Speaking as an online seller, here's a couple of things you can do to make everything come out right since almost everyone's computer monitor is different, and pictures can vary a lot in terms of how they represent the colors.
First, if at all possible, buy your coordinating fabrics from the same line as your focus fabrics. Buying your blenders from the same manufacturer's lines will often help make sure you get good color coordination as well. This will help assure you that you are getting good color matches, if that's what you're after.
Second, call the store owner and ask! If you see something you like, or have something that's on their website, and want to coordinate with it, there's nothing wrong with calling and asking. This may not work with larger online mega-stores, but most online shops are small operations that are happy to help you coordinate.
Third, if you're in doubt, ask for a small sample of fabrics you're considering. Obviously you need to be a little conservative about how many you ask for, but there's nothing like having a small strip or sample in hand to make sure that's what you need before you order.
If you're trying to coordinate with something you already have, why not send a little swatch to the store to have them try to coordinate it for you? Most small online shops are very happy to help you with your project.
Make sure that the online shop you're buying from has a fair return policy, just in case you totally don't get what you expected. However, be fair to the shop and try to do your share to make sure a return isn't necessary very often.
More people are going to be shopping online as fuel prices skyrocket making trips to the LQS more expensive. It pays to do a little due diligence when you're shopping online as well.