The most common fault that is seen in photos is a lack of definite white and black points.
The photo may have been taken in poor lighting conditions or scanned in with an incorrect scanner profile.
  It appears muddy and lacks life.
   It is important that we understand that the eye perceives colour in relation to the total image, not as an absolute value.
  It will take in the image, identify the white and black areas as reference points and view the rest of the image with respect to those data points.
Using Photoshop, (or colour correction in a RIP such as Wasatch), we can enhance the image by choosing Image (on top menu bar), Adjustments then Curves.
  The curve shows the shadows in the bottom left and the highlights in the top right with a curve between them.
  Take the top right hand corner and drag it along the top axis until some white appears in the image.
  Take the bottom left corner and drag it along the bottom axis until some black is evident in the image.
It's that simple!  How far you drag it depends on the degree of muddiness.
  That is the single most effective method of correcting colour problems.
Secondly, when that is done, you may find that the image is too dark or light.
  Correct this by dragging the midpoint of the curve up or down.
Thirdly, if the image has any sort of colour cast, the individual channel can be chosen and dragged up or down to give the desired hue.
None of these techniques is a substitute for starting with a decent quality image but may rescue a substandard photo.