1) Open the image in PS.
2) Click on Layer menu. Point to New and select New Adjustment Layer.
3) A dialogue box will appear and select Hue and Saturation in the Type window.
4) Another dialogue bow will appear and check Colourize box.
5) Slide along the Hue and Saturation bars until the desired Sepia effect. Hit OK.
6) On the Layer window, adjust the opacity of the adjustment layer until desired effect.
7) Merge the 2 layers together by pressing Ctrl+e.
Guzzler, are you talking full-colour here or just selective colour? if you wanna make a sepia photo into normal full colour, that's damn near impossible, cos the colour details are not there. only way i can think of is if you have a dam super good eye for colour and paint in the colours pixel by pixel (like restoring old photos, etc)
if you're talking about selective colouring, like making someone's face blue instead of sepia tone, then just use a little creative lasso-ing or magic wand in PS and then fill in the slected area with the colour you want.
Are you referring to hand coloring? Doing it manually on a print (with watercolor paints, etc) is quite a specialised art... try searching the web for "hand coloring" and you will probably get some useful resources. I've seen quite a number of hand-colored photos posted on sites like photo.net, so if you're really into this you may want to ask some of the photographers who do this sort of thing for advice. I presume a web search will also yield plenty of useful techniques for "digital" handcoloring too. Sorry for not being able to provide any advice of my own, because this is an area I have never personally attempted, though I certainly admire this form of art. Hope this helps you get started, though.