The color I see in my foreground selection is different to what I see when I add it to my artwork. I am working in CMYK, the mode is normal and the opacity is 100%. I haven't seen this before, so I am wondering if the client artwork that I am working on may have had some inbuilt settings that have superceded my settings (is that even possible?). I am using CS3 Photoshop on a PC, and the upgrading or modification of either is not an option. I appreciate the time taken to reply to this, so thank you in advance, and I hope that a: this has a fix, and b: it doesn't make me look too idiotic.
A document's colors are described by an associated color profile.
You can use the little status box at the lower-left of the canvas window to show the document's profile. Alternately it can be seen in the Info panel (with appropriate settings) and also it will show as the current profile if you choose Edit - Convert to Profile (cancel, of course, afterward if you don't want make changes).
What profile are you using?
The document's color profile will describe the gamut of colors that are possible for that document type. CMYK work doesn't normally have nearly the gamut you see when working in RGB.