Adding Color to a Cartoon

Let's take a look at a girl kissing a cartoon.



Her supposed "date" appears black and white, so let's make it more formal by adding color to it.



I have covered the guy's face with just color using another layer. This is because I do not want to remove his face by paining all over it in the same layer. The next thing I did was play around with layer modes (overlay, soft light, etc.) until I feel the result looks sufficient.
Also, I avoided coloring the girl because she is already in color! Oh, the significance of photos!



I chose Multiply for the layer mode because it gives the guy his skin color without overwriting the black lines; darken works too.



I repeated this process for the other parts (including the hair). The entire image now looks more detailed than before because of the color.