I love sketches. And surprises :)
Adding some colors with sometimes unexpected result is what I want to share with you.
Just open a sketch in Photoshop and chose Image -> Adjustments -> Gradient map. Play with options there and find gradient that suits your image the best. The result may be unexpected. If you like it, adjust some colors, options, add some strokes and effects to finish the picture.
You may also make some copies of the sketch layer, apply different gradient maps to them and then use masking to reveal some of one, some of another to enlighten some central figure or different picture parts.
If you use noise removal filters afterwards and maybe add some blur the sketch will turn into painting.
It's like a game. You can never know which gradient will add magic to you sketch. That's why I love it.