Colour Balance, Gamma, Colour Temperature
Take a look at this test image. Assuming that you have already set the brightness and contrast correctly. Left-hand section - You should see a gradual change of tone from black to white. Look carefully at the lighter greys, these should be neutral grey. If they are tinted at all you have a predominant colour (see "Colour balance" below). There may be some horizontal bands showing, if minor this may be normal, especially for flat screens, if serious you may have a problem with the compatibility between your graphics card and your screen - make sure that the make & model of screen is recognised by the graphics card and that the correct driver software is installed. Centre section - Pale skin tones are a good test of screen adjustment as your eyes are used to what it should look like. In this image the skin tone is pale and neutral (taken facing north light on a winter morning). If you see un-real coloration, blotchy pink, etc, this can be caused by incorrect Gamma and/or Colour Balance (see below). There is a slight blue tint (unintentional artifact) to the shadows on the curtain, otherwise it's neutral off-white. Right-hand section - This is a typical 'holiday snap' of the south coast of France, taken on a bright sunny afternoon. The colours should be well saturated, but not to the point of being un-real, nor being hazy or pale, It should look like afternoon, not evening. Assuming brightness and contrast are near correct, anything odd reveals problems with the Gamma setting (see below).
Gamma: What you see between the two extremes, fully white and fully black, is governed by the 'gamma'. The relationship between the signal supplied to the screen and the effect seen by the human eye is not proportional and if you were to plot it on a graph the line would be curved (see below). The amount of curvature is the "gamma". This is a numerical value, usually between 1.6 and 2.4.
Image: Norman Koren
If the gamma is too low the image will look 'washed-out' with a lack of depth and detail. If the gamma is too high the colours will be over saturated and perhaps over dark and the contrast will look unrealistic. This can be adjusted in the parameters of the graphics card or sometimes via an adjustment menu of the screen itself. You can make the adjustments, refering to the test image above, until it appears to be as natural as possible. Note that the normal setting on a Windows PC is a gamma of 2.2 and this is a widespread standard, particularly for Internet, although sometimes this parameter value is not correct compared with what the screen actually does, ie the value you need to set may be different to get the screen to look correct and like it should with 2.2 ( my screen needs a setting of 1.8 to give a "2.2 look" ).
Colour balance: The screen image is made by mixing three primary colours, red light, blue light and green light. If these are not mixed in the correct proportions the image will look tinted towards a certain colour (red, blue, green, yellow, magenta, etc.). The screen may have a predominance in one or two of the primary colours and this needs to be compensated for in order to correctly display an image. If the white that you see is tinted, then the relative strength of the three colours needs adjusting. This is usually possible in the parameters of the graphics card (see image below) or sometimes via an adjustment menu of the screen itself.
If the white is white, but the mid-range greys look tinted then the relative gamma for each of the colours needs adjusting. In some computers, the combination of graphics card / screen / driver software allows the gamma to be adjusted for each of the primary colours individually (see below). Adjusting the gamma in this way does not alter the colour of the white, only the mid-range colours.
Colour temperature You may have an adjustment available for 'Colour Temperature' (either as shown above, or as a number XXXX K). This sets the overall colour slightly towards warm (red) or cool (blue) and this is particularly noticable in the whiteness of the white and in the pale colours. This should typically be set between 5500K and 6500K, although this may not be totally reliable. (With my previous graphics card, my screen needed to be set at 12250K to look like 6250K !).
Screen calibration programs: There are many simple calibration programs available which help in setting your screen correctly. These may be supplied with the screen or with graphics/photo programs. For example, "Photoshop" provides "Adobe Gamma". There are also similar adjustment methods available on Internet. These usually involve matching a plain coloured square to a similar square with fine lines on it, for grey and for each of the 3 primary colours. My own experience shows these to give widely variable results, even when you have fully mastered the technique (not easy at first). After limited sucess with these, I would suggest that better results can be obtained using the test images that are provided here. If you need to ba absolutely sure of correct screen calibration, then buying a calibration probe and real screen analysis software is well worth the small investment. (See "Spyder" on this site, from £ 49.)
Colour management: The subject of colour management is vast and it is not possible to enter into it here. Bear in mind that if you have any 'colour management' activated on your computer such as matching your screen to your printer, then images that you see on Internet or photo files that you prepare for lab printing may be unintentionally 'corrected' if the colour management is not correctly used. An issues such as this need to be resolved before attempting any calibration of the screen. The colour management needs to set to 'Windows default', or 'Internet' or completely disabled whilst setting the screen. Useful links: Colour management in Photoshop