I have a document header in a single color (red) that I need to convert to "1/color" - not sure what that means but my question is how can I do the conversion in photoshop?
CMYK colors can be converted to 100% one color by changing them to corresponding Pantone color (Spot color). Pantone colors are single colors basically. There will exist a Pantone color corresponding to all possible combinations of the CMYK colors. You can match the color of your document header to one of these pantone colors and apply it.
IFrom the date of the original post, this may be to late but we should make this a mantra in a Typesetting forum - Never Set Type In Photoshop. The basics of file size and resolution dictate this. Excepting headings, where one just has to have a specific effect, Photoshop is not a typesetting program. Can you get a spot color? sure. The easiest and most reliable would be to create a grayscale image and apply a spot color in the page layout software. If you needed to utilize PS, understand that a color is a channel and read the PS help in regards to spot channels. Create a spot channel, work in that channel with the element, and save a DCS eps file. Depending on your version of PS and the recipients software, this will raise some eyebrows at the least. (Expect them to try to open the file in Ilustrator and inform you that the eps is corrupt)
Also not a good idea to do a true spot color ad in photoshop for newspapers that deal in spot color. You are better off bringing the greyscale photoshop document into another application and then add color.
You could define your red as a spot color and have a spot plate.
I find it easier to just use one of the CMYK plates for a second color (in your case, I would use the magenta plate) and then tell the printer that the second color is Pantone such-and-such. This lets you spec any Pantone color without having to modify your document.