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An Experiment with Colour Overlays
the reference photo is at the foot of this page, the three moveable blocks are:
1. lowest layer black and white which is the reference photo converted to black and white in Photoshop using 100% on all the colour channels with the default preset.
2. middle layer green, I use the Photoshop channel mixer to set the red channel and blue channel to zero using default preset. Then set that layer to 65% opacity
3. top layer red. I use the Photoshop channel mixer to set the green channel and blue channel to zero using default preset. Then set that layer to 40% opacity.
Each of these layers is saved in png format to preserve transparency.
The red and green transparency overlays produce an approximation to full colour, which is quite surprising, given we’re using a subtrative mix, or I think we are.
Just clicking on the black & white, and red, checkboxes gives a very marked demonstration of colour constancy. The greeny-blue you see if entirely in the eye, if you colour-sample the pixels you'll find they are grey (i.e. r=g=b).
black & white   green   red
 
black and white
red
green
reference pic
 

 
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