I'm sure you've seen it before. For example a photo of a person, completely black and white, except the lips or his tie. Or a photo of a fruit basket whit only one red apple.
This effect is easy and takes just a few seconds with Photoshop!
Find a nice picture that is suitable for this effect (that's the only downside, not all pictures are suitable for this effect). For example this picture of a sheep.
Turn the picture to black&white by using desaturate (this is not the best way to turn a picture to b/w, but it is the easiest!). Press Ctrl+Shift+U to apply this effect or select it in the menu:
This is the result:
Select the history brush (Y):
Make sure your "history palette" is visible:
After this, select the history state before the desaturation (in my case a resize action, but this can be everything):
Zoom in on the piece you want to be coloured and "color it" with your history brush. You'll see the colors of the original picture "coming through". Make sure you don't use a hard brush that is to big, because it is very important to color between the lines. If you've done it all correctly you should get this: