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".
- Open the "Color and Tone" dialog and apply the "Channel Mixer" command (see screenshot).
- In the dialog box that opens, select the proper channel in the "Output Channel" drop menu. For example, to alter the blue-to-yellow ratio, you select the "Blue" channel.
- 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% (screenshot).
- In the Layers palette, select the front (uppermost) layer.
- Press [Alt+Ctrl+Shift+E] to merge all visible layers to a new layer.
- Edit the created layer with "Channel Mixer".
NB Using the Channel Mixer for B&W conversion explained here.