Make sure you are not in grayscale mode. It should tell you at the top of the screen which color mode you are working in. It is possible to add colors to the image, but you can not restore the original colors once the colors have been removed.

This should help:
Create a new layer on top of the black and white image, and set its blending mode from normal to color. Select an appropriate color and use your brush tool on this new layer.

Once the color is gone, there is no way to convert it back. It's kind of like crushing a car that way. You take one dimension out and there is no getting it back.

I hope you are working on copies and not the original files. If you save the grayscale over the color original, you are stuck.

a)open the image in photoshop
b)image>mode>rgb
c) duplicate the layer ctrl+j
d)color the layer , if needed mono color , select the color in colour pallete and then just use shortcutkey ctrl+u and adjust
e)if needed proper color correction then use pen tool to select the needed parts and save in selection,then color it based on your wish and taste

Hope so my answer helped you