Change a black and white Tiff to a Pantone 802 color

Grayscale mode to duotone mode with monotone option.

There are a couple of ways to do this.  You can select the greyscale channel and then copy it.  In the channels tab Click on the lines to the right of the top and select New Spot Chanel.  Click on the color in the dialog square that comes up and click on Color Libraries  Select Pantone Coated or uncoated depending on the paper it prints on, then select the PMS 802 in that dialog box.  Click on that Pantone 802 chanel and past the copy of the greyscale chanel in that.   Then select the grayscale chanel again and select all and fill with white.  Make sure only the Gray chanel is selected and fill that with white.