I suggested a possible change to a Flickr friend’s photo.
You can see the original here.
I thought it would look better fully monocrome with just the lips and the flowers in their original colour.
Here is how I did it.
Load the original picture into photoshop.
Added an adjustment layer (the black and white circle at the bottom of the layers pallette.
Choose Channel Mixer
Tick Monochrome and fiddle with the sliders until you are happy with the image.
The whole image is now black and white, how to get the original colour back?
Next to the later thumbnail there is a layer mask, currently all white, white means that the layer effect will be seen.
Click on the white layer mask.
Select the colour black and a brush you are happy with and paint in black on the picture the areas you wish to be in colour.
When you paint you do not see black, what you see is the effect of the adjustment layer being masked out, so you “see through” the black and white that has been added.
You can see my version here