When colors in an image are shifted towards a color that isn't intentional, this is considered to be a color caste. Fixing a color caste using the Curves palette will be very intuitive for those who are familiar with mixing colors of paint. For example, when a paint color is too red, you can make it less red by adding green, or by reducing the amount of red. The same is true when an image is too red.

Step 2: Click in the middle of the curve to add a control point. Drag it up or down, depending on the requirements of the image. For example, if the image is too red, you can target the Red channel and reduce the red by dragging downwards. If the image is too green, you can reduce the amount of green by dragging upwards in the Red channel, or by dragging downwards in the Green channel.

Every image will be slightly different, and will often require slight adjustments in all three channels.