Looking around the Art Camp 2 Week 5 stuff I see a lot of people apologizing about their colours being off in their traditional master study paintings and other paintings that are referenced from photos. Here is a bit of advise to help you out in matching the reference colours traditionally:
Make a colour copy (on paper from a copier/printer) of the image or print out a colour copy if you are looking at something online, make sure the colours come out as you want them to be. Now either when you are setting up your paint colours to use or if you mix them while you work take a small daub of the colour and paint it on the colour copy in the area you are trying to match. If the colours blend and you can't tell the difference between them you are good to go. If the colour is off try again until they blend into each other. Just make small dots on your colour copy to test your mixed colours so you leave plenty of the actual colour. And that's it! As long as your drawing is accurate and you test your colours to get them to match then you can come up with a pretty close replication of what the image is. You can even use this to get accurately matched colours for your own Illustrations if you have good reference you can use or are just trying to get a specific colour.