How would one pick or select all of a single color within Photoshop, Paint.NET, or The Gimp?
This is a vague question so I'm just going to answer it with what I've used before.
You would use the eyedropper tool shortcut I.
You would use the color picker tool shortcut O.
Also called the Color Picker Tool
Imagemagick command line:
Gimp - Select by Color
In Gimp we have the Select by Color tool (Shift+O) to allow a selection based on image color.
Select by Color - Threshold
To select a color, the Photoshop approach is to use
If your color is very well-defined and you don't need an anti-aliased or soft-edged selection, you can use the Magic Wand tool. Set the sample size (which you have to do via the Eyedropper tool in earlier versions of Photoshop) to 3x3 or 5x5 average, and click on the color you're trying to select.
If all you want to do is sample the color, then the alternatives Gramps outlined are correct, although I would again suggest you set the sample size to something larger than a single pixel if the image is jpeg compressed.
(the principle should be the same in other programmes)
I might have misunderstood, but you are trying to select all fields of one specific colour in an image?
Select an area with the colour you want, fiddle with the
Right-click on the area you have selected, and choose