It's not an easy thing to do. Since we don't all use color corrected computing equipment, the shade that I see on my monitor won't be the same as the stamp is in real life.
We can see the differences between stamp shades when many of them are taken in one picture or scan as you've done. We can't tell you exactly what you have, but we can say that one shade looks close to a certain color.
When you've got a bunch of differing stamps and have some that only come in one color, then you'll be able to identify the different colors. The reds are particularly difficult unlike blue-green, green, or yellow-green for example.
Color guides for each issue can be purchased and are pricy. The named shades are similar between issues.
Here's some pics to give you an idea of what the different shades look like. I copied the first one from an
ebay auction and have no idea how this is organized nor if it's correct:


From essayk:
