It might seem hard, but it isn’t. However, it is well time consuming.
However, I deem some parts of this tutorial to be more advanced,
it might require some previous knowledge of Layer Mask or Quick Mask .

1. Open Grayscale image

2. Layer/New Adjustment layer/Hue Saturation

3. Next to the layer, there is layer mask, which shows what part of the picture is affected by the layer or what part of the layer is visible. White means visible, black the opposite.

Click on the Layer mask and fill it with black, so that no effects will be shown. Use brush tool to paint white in the layer mask. After doing that, double click on the Adjustment layer to change the settings. If you are not happy with the layer mask, just click on it again and paint in it either black or white.

Do the same for lips (make another adjustment layer, paint its layer mask black and draw white in the position of lips). Repeat the same fo all parts of the picture, hair, eyes, clothes and skin (this is the worst part, cos it is tremendously difficult to find rather natural hue).

Try and have fun. If you can do this, you can be proud of yourself for this process needs both imagination of an artist and precision of an artist.

Source: http://www.t-tutorials.com