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http://photo.stackexchange.com/questions/15495/how-do-i-select-all-pixels-that-match-an-exact-color-value-in-adobe-photoshop

You can use the Magic Wand Tool with the following setting:

Tolerance: 0
Anti-alias: unchecked
Contiguous: unchecked
Sample All Layers: *checked or unchecked* (See below)
The Magic Wand Tool also uses the "Sample Size" settings from the Eye Dropper tool, so you will have to switch to that tool and make sure it is set to "Point Sample" before using the Magic Wand Tool. Otherwise you could end up selecting more colors than you intended.

To select a color by value, the easist way is probably to make a new layer and fill some part of it with the color you wish to select, and then use the Magic Wand tool (set to sample all layers) and click on your color patch on the new layer.