Duotone will give you the spot color over the entire image. If you want to colorize just a part of the image, you gotta think channels. Essentially you will be making a manual separation.
First of all, find out if your printer can handle DCS (Desktop Color Separation) 2.0 files. If so, add a spot color channel to your grayscale image. Specify the color assigned to that channel. Copy or cut (depending on the effect you wish to achieve) the pixels you want to print in the spot color to the spot color channel. When all’s well and good, save as DCS 2.0, single file, color composite. Place in your layout and send to the printer.
If your printer doesn’t take DCS 2.0 files (In a PDF or composite workflow, only the 72dpi DCS 2.0 preview is used. Bad news.), the quick and nasty workaround is to spoof your spot color using CMYK mode. First select all in your grayscale doc, copy and create a new CMYK file (by copying to the pasteboard, you tell Photoshop the exact dimensions of the file you’re about to create.). Paste the grayscale photo into the BLACK channel. Copy or cut the pixels as before, but paste them into the MAGENTA channel. You should end up with the red stuff in the Magenta channel, the black stuff in the Black channel, and nothing in the Yellow or Cyan channels. Save as TIFF or EPS (or PSD if you’re placing into InDesign) and place into your layout. Tell your printer to output only Black and Magenta and tell him to print the Magenta plate in the spot color you choose. (Consult a fresh Pantone guide to select the ink.) As always, communication with your printer is vital.
