There are two main colour spaces which you will find in most design software. These are CMYK and RGB. CMYK stands for Cyan (C), Magenta (M), Yellow (Y) and Black (K), and is for print use, whilst RGB represents Red, Green and Blue, and is used for screen or web applications.

All artwork for print, from flyers and posters through to business cards and stationery, MUST be in CMYK colour format, as these are the colours that combine to create full colour print. Any artwork that is supplied in RGB will be converted to CMYK, but please note that the colours may change slightly when this conversion is carried out. Brighter colours will be the most affected. Ideally the artwork should be created in CMYK format from the outset, rather than being converted later in the process.

If printing in black and white, Greyscale colour format is also acceptable.

How to Select / Convert Colour Spaces

Fortunately it is easy to change colour spaces, but keep an eye out for any colours that change in the process.

Photoshop

If creating a new file from scratch, select Colour Mode > CMYK in the New Document window.

If for any reason you need to change the colour mode after beginning your artwork, simply select from the “Image” Menu at the top of the screen: Image > Mode > CMYK Color. Your document is now in CMYK.

Illustrator

If creating a new file from scratch, select the “Advanced” tab in the New Document window, and select Colour Mode > CMYK.

If you need to change the colour mode after beginning your artwork, simply select from the “File” menu at the top of the screen File > Document Colour Mode > CMYK Your document is now in CMYK.

Indesign

In Indesign, the colour mode is defined when you output a PDF or EPS. If exporting a PDF, Select File > Export. Then select PDF from the drop down menu and name your file. When you click save, you will be directed to the “Export Adobe PDF” menu. Select “Output” on the left hand menu and set the destination as “Document CMYK – U.S. Web Coated (SWOP)”

If exporting as an EPS, select File > Export. Then select EPS from the drop down menu and name your file. When you click save, you will be directed to the “Export EPS” menu. Select CMYK from the “Colour” Menu. Your document is now in CMYK.

Quark

In Quark, the colour mode is defined when you output a PDF or EPS. If exporting an EPS, select File > Save page as EPS. Click “Options” in the Save page as EPS window. Select “Colour” on the left hand menu and then select “Composite CMYK” in the “Setup” drop down menu. If exporting a PDF, select File > Export > Layout as PDF. You will be directed to the “Export as PDF” window. Select “Colour” on the left hand menu and then select “composite CMYK” in the “Setup” drop down menu. Your document is now in CMYK.

Additional Resources
Wikipedia: Colour Space
Converting to CMYK (incl. instructions for PageMaker and CorelDraw)