I tried gcolor2, but wasn't very fond of it. gcolor2 included a bunch of functionality I didn't need and not enough of the functionality I wanted. Perhaps the functionality with the largest weight on my decision making was the magnified section provided by Colorpix for making sure I get the color for the pixel I want. It might seem obvious in some instances when you're over the color you want without magnification, but alot of times I'm looking for the color of a single pixel wide line & it's nice to know I need to move left one pixel instead of moving right a few pixels before realizing I'm going the wrong direction.
My other alternative, was using colorpix through Wine like some bloggers have mentioned here and here.
The only problem I had with that is I don't always have Wine up and running, so it can take a few seconds to load.
So, since I'm trying to pickup C/C++ I decided to write my own implementation of Colorpix to run natively on my Ubuntu system.
So far it starts instantly VS waiting for a translator to load, displays the color in RGB and hexidecimal format, and magnifies an 11x11 pixel area around the pointer 2000%
I've made the source available via a project named Picksel at Google Code.