You can restrict blending effects to a specified channel when you blend a layer or group. By default, all channels are included. When using an RGB image, for example, you can choose to exclude the red channel from blending; in the composite image, only the information in the green and blue channels is affected.
Double-click a layer thumbnail.
Choose Layer > Layer Style > Blending Options.
Choose Blending Options from the Add A Layer Style button at the bottom of the Layers palette.