Please consider this image:
The color in the top swatch, labeled "original color" is made from an image I downloaded from the web. I cut out a bit of a colored area that I found appropriate, scaled the image to a size of 1 x 2 pixels, to average out the graininess of the image, then scaled it back up to get an average hue. According to the color picker tool in GIMP, the hex value for this color is 485938, but in the image it is actually 445735. The color in the second swatch is what I obtained when I set this as the foreground color in GIMP, and according to the GIMP color picker tool, the second swatch is, in fact, 485938. The color in the third swatch, labeled "Blender representation", is what showed in the diffuse panel of the Material Properties editor when I entered the same hex value as a hex value in the Blender Color chooser; however it displayed in the diffuse panel as 51a83f. The green color in the fourth swatch is the result when this diffuse color was applied to a series of objects, and rendered using Blender Internal; the color from the render is 2a5c20.
How do I get the bottom swatch to match one of the top two?