The GIMP is a free open source image editing program with capabilities that match those of Adobe Photoshop and offers some facilities not available in Photoshop. The downside is that the GIMP is not as easy to use and the documentation is poor. The rendering engine is subtly different from that of Photoshop and the interface can be confusing. Both these facts may confuse the beginner. Nevertheless after a while it grows on you and you realise it is a very powerful tool.

This tutorial was produced using GIMP 2.6.2 for OS X. The Windows and Linux versions should have the same interface.

As a lover of colour ( Maybe I should rephrase that but won't ) I am constantly amazed by the fact that black and white pictures may have more power and impact than the original colour version.

The Gimp offers several ways to convert a colour picture to a monochrome picture: a picture that is either black and white or shades of a single colour including: Desaturate, The Channel Mixer, Colorize and the Hue-Saturation control. All these are found on the Gimp's Colors menu.

To start copy your image and drag the copy onto the GIMP icon.

This is the simplest option. There are three options in the Menu. Lightness, Luminosity and average.

Try each one and see whether you notice any difference, and if you do choose whichever you prefer.

Generally speaking I prefer the luminosity option. The advantage of simplicity using Desaturate is counterbalanced by the lack of control over the process and the fact you will probably have to use the dodge and burn tool to create a good picture.

I noticed incidentally, that the “Black and White” pictures in photo magazines often looked more of a chocolate colour and I got a more pleasing result by boosting the red with the colour balance menu

This option lets you change the saturation of all colours or any one of the ranges corresponding to the three primary colours and the three complementary colours. These are arranged in a colour circle. If you increase the hue the colours move clockwise round the circle and anticlockwise as you decrease the hue. If you do not remove the colours entirely you get a subtle effect which can be rather like hand colouring a print, and in any case leaves a trace of colour ( and warmth) in the picture which offsets the coldness of a grayscale picture

This menu lets lets you change the hue of a picture and the saturation of the result.
The effect is easier to control than with the Hue-saturation menu and can be very pleasing though sepia is hard to achieve.

This is  a sub menu of the Components menu. Check the Monochrome box and optionally the preserve luminosity box then adjust the sliders till you like what you see. The best effect  normally arises if you adjust the red first. Ideally all the  slider values should add up to 100%

Choosing image->Mode-> greyscale produces a result that is  close to the ideal channel mixer result but  does not do the same as  Desaturate.  The balance of  the RGB channels is different.

The GIMP offers a number of ways to convert a  full colour image to a single colour or grayscale image, These  all produce different  results and  what works for one picture may not work for others. When converting an image  be ready to try any or all of these methods. Always work on a copy not the original.