The image on the left has a severe yellow and cyan cast that is causing it to look muddy. The sky looks more neutral than blue, and the skintones are too heavy in the cyan plate. The image could also use a serious injection of contrast.

Remember that as a rule of thumb, neutral areas must have equal amounts of magenta and yellow and then a few points more in the cyan plate.

Skintones, on the other hand, should have roughly equal amounts of magenta and yellow and then a fifth less of cyan.

Let the info palette in Photoshop be your guide. Everything should be done using the numbers.

I attack the yellow plate first because correcting it will make the greatest positive impact on the image. Color cast is always most apparent in the highlight and less so in the shadow. In this example I've removed yellow ink from the highlight, mid-tones, and 3-quarter tones, leaving it in the shadow in order to keep the trees from turning blue.

The majority of the images you will encounter will need a cyan plate adjustment. This is due to the nature of light itself. With the exception of early morning and dusk, most outdoor light reflected off our subjects is blue cast. In addition, lenses, scanners and the inherent nature of film also have a tendency to introduce too much blue. A typical cyan cast can easily be removed by placing a point in the mid-tones area to keep it put and then pulling your quarter-tone down till you reach the desired level. In this example I've decided to bump up the cyan in the 3-quarter tones.

As a consequence of removing cyan and yellow from the image, it is now necessary to remove enough magenta to normalize the image. Doing this will bring my neutrals and skintone where they need to be. Notice how I have blown out the highlight. In newsprint the highlight shouldn't contain any ink. Keep in mind that you are adjusting ink amounts on the press rather than color on your monitor.

Finally, in the CMYK channel I do an overall "steepening" of the curve to introduce some contrast to the image. Most of the time this is a small adjustment done to add snap to the image.