Let's start off with a typical request. Clients often take images used for color or magazine ads, then repurpose them for use in color or black and white newspaper ads. Typically all retouching would have been done to an image already, so all that's left is to prepare the image for newspaper specifications so it will print well.
There are two typical scenarios you'll encounter when working with a color file: a client may ask you to take an image and spec it for use in a color ad in the newspaper, or you may be asked to convert the image to a grayscale image to be used in a black and white newspaper ad.
In either case, assuming I receive a color image to spec for newsprint, I always make sure I spec both a color image and a grayscale image for newsprint, regardless of what I was asked to do. I say this because there have been occasions when the client has asked to spec a supplied color image that will be changed to a grayscale image, only to come back at a later date to say that they now require a color newsprint image! It just makes your life a little easier if you have both on hand. Having a color image spec'd for newsprint also makes the conversion to grayscale a little easier.
Let's assume you have a CMYK image. (If you have an RGB image, I'll discuss the transformation to CMYK shortly.) Start off by making sure the Information palette is open. Open the Information palette preferences, bringing up the dialog ...