Black and white photography has still a lot of supporters, in spite of current situation with ultra-modern cameras, automated settings, 3D, HDR... But not everything must be grayscale. There are multiple techniques for the late colouring of black and white pictures. Among them, I want to remark the layer multiplication technique, really easy to apply in Photoshop or other edition programs, like the freeware GIMP.
Colour pictures (JPG, GIF, PNG...) typically store image information for each pixel in three 8-bit values, each one responsible for a primary colour (Red, Green and Blue).
With eight bits per channel, each colour will provide up to 256 different intensity levels or colours. Between all three channels, an image can include more than 16,7 different million colours.
The "multiply" function
In edition programs with layer handling capabilities, the multiply function does just that: To multiply the values of two specific layers, per pixel and channel. The result is then divided by 255, to reach a final value in the valid range (0 - 255).
Since 255 is the maximum possible value, the result will be usually darker than the original layers. In RGB images, final colour will depend on the reference colours from both top and bottom layers.
In the case of black and white images, the original (set in bottom) will have exactly the same information on all three channels, so the colour information will be provided by "the other" (top) layer.
- Convert the black and white image to RGB, if it is in grayscale. Use the command "Image - Mode - RGB"
- Add a new layer, by clicking the right button over the image miniature in the auxiliary layers window and choosing "New layer..."
- Fill this new top layer with the desired colour (you may pick it from the tools window, or with the colour picker from another picture). In this step, you will not see the lower layer anymore.
- Choose fusion mode to be "multiply" from the drop-down menu in the layers window. It is important to have the top, coloured layer as active for this step.
Main davantage for this method is that it is not a destructive one; you can always recall the original black and white image just by hiding (or removing) the top layer.
Furthermore, you may produce different versions of a same image, just by changing fill colour: sepia, blue, magenta... any colour can do.
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