Yes and no.
There are various comparison guides online, but most simply use the hex code given by the company. In most cases the coloured box on the screen is similar to the actual colour, but not always. There's also other factors to take into account, like whether the paint has been applied over a dark or light (or gradiented, or coloured...) undercoat, which will change the final result. Also your screen settings can change the way it appears on screen, too!
Here are some examples for you though, just don't forget that they won't be perfect. =]
Also worth bearing in mind is that colours don't have to be perfect to a guide. If you're painting Ultramarines you don't have to use whatever Citadel call Ultramarines Blue these days (is it Calgar Blue now?) - VMC Flat Blue is a very different shade, but you can still use it. Everything is open to interpretation, and GW change things themselves! You'll remember that Ultramarines in particular user to be much brighter than they are now, their yellow has changed to gold, and every other company colour seems to have vanished!
