To apply the characteristics of the red channel into the entire image, we need to replace the luminance of the entire image with the image from the red channel.
Open an image you would like to work with or use the vancouver.jpg file we still have opened in Photoshop.

Duplicate the current layer (Layer> Duplicate Layer or Ctrl+J).
Go in to the Channels pallet (Window> Channels) and click to select the Red channel.

With the red channel selected, create a selection of the canvas (Select> All or Ctrl+A) and copy the selection to clipboard (Edit> Copy or Ctrl+C).

Before we paste the red channel into the Lightness channel, we need to convert the image into Lab Color (Image> Mode> Lab Color). When it asks "Changing modes can affect the appearance of layers. Flatten image before mode change?", select Don't Flatten.

Go in to the Channels pallet (Window> Channels) and click to select the Lightness channel.

Paste (Ctrl+V) the red channel that you have copied in your clipboard. The Lightness channel should be replaced with the image from the Red channel.
