A colour mask is a term used in Photoshop to refer to an image in which part of the colours have been replaced by black and white. I.e. only a portion of the image remains in colour.
Colour masking can be utilized very effectively to draw attention to a particular portion of the image alone and is used extensively by photo editors all over the world.
A few examples of situations where colour masking is used are:
- A photograph with a bunch of people in which attention is drawn to 1 or 2 people alone by masking the rest in black and white
- An advertisement in which the model alone is in colour, while the background and surroundings are in black and white
- An advertisement in which the product alone is in colour, while the background and surroundings are in black and white
Note: There are many ways to do this in Photoshop, and one such way is described here. However, the same effect may be achieved by a totally different sequence of actions also.
1) Open the image in Photoshop.
2) Add a Hue/Saturation Layer by Clicking on “Create new fill or adjustment layer” in the layer’s palette.
3) Make the Saturation Value equal to -100. I.e. move the slider to the extreme left.
4) Click on the erase tool and drag it over that portion of the image on which you would like the colour to be shown.
5) If by mistake you erase some of the portion intended to be black and white, then switch foreground & background colours, and drag it over that region again.
6) For fine edges, use a round brush of a smaller size instead of the square shaped erase tool.
7) In case of any errors, switch foreground & background colours to correct the error, or use the History tool to go back a few steps.
8) Repeat this process until the desired effect is achieved.
For further reference, Beginners can refer this article on "Introduction to Adobe Photoshop CS3".
The above technique has been applied to the images shown below. The “Before” and “After” stages are both indicated here. The initial image has been desaturated completely with a Hue/Saturation Layer, and then a brush has been used to bring back colour to the flower.
