joe do this:
start with colour RGB picture.
image menu/mode/grayscale/grayscale
if you want a sepia colour do this:
Ok with your grayscale picture.
image menu/mode/duotone
in dialog box that appear leave colour 1 as black and choose a yellow colour for colour 2
Ta da picture is coloured.
before you can save it as a JPEG you must convertthe picture back to RGB.
image menu/mode/rgb
Hope this helps, if you need any more help just post your question, been useing photoshop for almost 10 years now !!