Ideally, the monitor that you use for image processing should be calibrated to ensure accurate reproduction of colour in your images, both on-screen and for printing.  To do this requires a colourimeter - probably the best known is the Spyder - to measure the monitor's response and allow you to make adjustment to achieve accurate output.  Sadly, many/most of us do not have access to such a device.  But there's some things that you can do to check, and possibly improve, your monitor's performance which do not require any specialised equipment.  The best site I've found for this is the set of LCD monitor test images at Lagom.nl.  Give it a try - it's free, and could prove to be very useful.