Getting a color version from a grayscale one is a pretty simple job for simple objects. But When its something like fire you won't like single colored fire. Thats where this tute kicks in.
http://thumb4.webshots.net/t/53/653/0/24/44/2781024440098553724HltgVv_th.jpg (http://entertainment.webshots.com/photo/2781024440098553724HltgVv)
Original Image
Extract the fire area to a new layer. I used the blue background to enhance "viewing pleasure"
http://thumb4.webshots.net/t/58/158/3/3/75/2529303750098553724whfqTP_th.jpg (http://entertainment.webshots.com/photo/2529303750098553724whfqTP)
Extracted Image
select the extracted fire layer and duplicate 2 times using ctlr +j. Name
the lower layer as yellow (optional) and the upper one as red (optional again).Select the yellow layer.
For CS2/3:
Click on the small ying-yang type icon in the bottom area of layers palette and select Hue/Saturation from the context menu that appears.
For pre CS2:
Press ctlr+U to bring Hue/Saturation dialog box.
Modify the dialog box to get following result.
http://thumb4.webshots.net/t/59/759/3/68/88/2852368880098553724ZboIUl_th.jpg (http://entertainment.webshots.com/photo/2852368880098553724ZboIUl)
Layer palette
Apply similar on red layer albeit with different settings.
http://thumb4.webshots.net/t/57/457/2/77/12/2038277120098553724HJMVuI_th.jpg (http://entertainment.webshots.com/photo/2038277120098553724HJMVuI)
Layers palette
(This step is required only for CS2/3 users) select the Hue saturation layer above Yellow layer and ctrl + click between the current layer and the one below it (i.e., H/S layer and Yellow layer)
Apply the same for red layer too. (see screenshots)
Now select the red layer and change its blending mode to color burn.
Here's the final result.
http://thumb4.webshots.net/t/57/557/8/76/55/2949876550098553724yaHjxI_th.jpg (http://entertainment.webshots.com/photo/2949876550098553724yaHjxI)
PS:
I did the masking very roughly. It needs more working for a real job.
I chose just 2 colors and a single layer was blended (color burn). For advanced effect more colors and other blending modes must be chosen. Including some changes in opacity and/or fill.
[Photoshop] Fire: B/W to color 
