Seeing Your Full-Color Image While Editing A Channel
When you’re working on an individual channel in the Channels palette (under the Window menu), by default Photoshop displays your current channel onscreen in grayscale. If you make changes to that channel, you only see how the change affects that channel. However, there is a little-known trick that lets you see the full-color image, while editing the currently selected channel. While you’re editing the channel, just press the Tilde key (~), which is right above the Tab key on your keyboard, and you’ll see the full RGB preview as you edit.