Depends on your monitor settings. I've found most fabrics I've bought online were pretty true to the color.

If you need an exact match, your better off going to your favorite fabric place to match it. If you are just ordering generally, the ones on line are usually pretty close. If anything, I found the colors to be sharper when I received them. Only one time did I get one that was way off, but it was pretty and I ended up using it anyway.

I've never had a problem but my mom has- especially ordering solids. I do have a more expensive high def laptop screen and hers is just a normal HP monitor, so maybe that makes a difference. We have put them next to each other and colors on her screen look darker than on mine.

I also agree with checking the ruler thing- I just ordered a bunch from Fabric.com and made sure to check them all. I like the huge scale fabrics myself.