If you have a color image in Photoshop, and you change to grayscale, a warning comes up asking if you want to discard color information. That means, once the color is gone, it's gone! More advanced users have even more complex methods of creating black and white images, so there is no way a plugin can know a certain shade of gray relates to a particular color. One shade of gray could just as easily be a red, green or blue!
If you want to color a photo, you'll have to do it by hand. One way to do this is to create a new layer, change the blending mode from 'normal' to 'color', and paint colors directly onto that layer. It won't be perfect, and you'll probably have to do a lot of adjustments afterwards, but it should give you a good starting point.