Channel Mixer in Photoshop Elements 5 and above

The "Channel Mixer" command lets you adjust an image by re-mixing its color channels. To use it efficiently, bear in mind the three sets of complementary colors: "Cyan - Red", "Magenta - Green", and "Yellow - Blue".

1. In the Effects palette, double-click the "Channel Mixer" thumbnail.
2. In the dialog box, choose the proper color channel from the "Output Channel" menu. For example, to alter the blue-to-yellow ratio, you select the "Blue" channel.
3. Try to adjust color with the "Source Channels" sliders, keeping your eye on "Total" percentage. For example, to fix a blue cast, you reduce this value; to remove yellow, you push it over 100%.

In Photoshop Elements, the "Channel Mixer" can edit only the current layer. To adjust a layered image as a whole:

1. In the Layers palette, select the front (uppermost) layer.
2. Press [Alt+Ctrl+Shift+E] to merge all visible layers to a new layer.
3. Edit the created layer with "Channel Mixer".

NB Using the Channel Mixer for B&W convertion explained here.