There are lists of colors by RGB value, and by Hex and access numbers.
So if I look here at Cloford.com
http://cloford.com/resources/colours/500col.htm
and pick, oh let's say, "palevioletred3" as a color I want to use in this post (I don't care about you going blind!) I can choose the Hex number (including the #) like this:
"palevioletred3""palevioletred3"
And there you have it. Pale violet red text, the third. Every girl's dream.
I can do the same thing in GIMP.
Click on the color box to open the Change Foreground Color dialog window.
You may add any of the three kinds of input - Hex or "Access" data goes in the HTML notation field, OR the RGB values go in the selection boxes after the three lowest color sliders (see the brackets on the left.) Once one kind of input is made, all of the other values match up.
Please note that in my screenie the website is in the background and the GIMP windows are in the foreground. Hope it isn't too confusing.

On to another color.
Look at the website (linked above) and check out the top of the chart.
On the first line you see Indian Red - the hex number is #B0171F and the RGB value is 176 23 31. You plug the RGB values into the right slots in the color selector and you are off and running with whatever lovely named color you wanted - in this case Indian Red.
Of course many colors are a matter of taste. I wanted a sepia-tone and googled "RGB sepia values" and found a variety of answers, but that's okay. It gave me a better starting off point than clicking until I thought I was close and then playing with sliders.
If you don't like this website there are others. Just Google RGB Color Values or RGB Color Values Chart or something, and check them out. But it is nice to know the info is out there.
/me shrugs
You artsy folks are laughing at me but I never knew how Penta got all those RGB color values for forum titles, so I just copied 'em. But now I can go schmooze 'em for myself.