Land was not the first to demonstrate the effect of creating coloured images from black-and-white film taken through filters, In the late 19th and early 20th century there were the Biocolour process, Kinecolour and early Technicolor, all used red and green filters onto black-and-white film, re-projected through red and green filters to create something approaching full colour on screen.

Can we simulate that on the computer monitor? On this page you select a colour photo and the code creates a greyscale image from that; it then creates three monochrome images in different colours, with translucency. Then it overlays these images which one would imagine would produce a subtractive mix – unlike projected images which form an additive mix.

The overlays do not produce a subtractive mix; it must be additive because although the amount of light is reduced with every overlay, the print process colours of cyan, magenta, yellow and black produce no better a result than red and green filters. Also intoducing the blue filter with red and green intensifies the yellows! So it is some sort of additive mix, but a rather dark one.

The colour overlays are made through the method of keeping the red, green and blue sliders in unison, there can be no brightness greater than the highest-numbered slider reaching 255, or, ignoring those that are on zero already, the lowest-numbered slider reaching zero. I also find that you really need a red or orange-red level 1 colour, and a green or cyan level 2 colour, other equally-spaced color values don’t have the same effect, though have some surprising effects nonetheless, for example in purple over black-and-white, the yellow areas of the original photo have a green look, magnified if a yellow overlay is included. These are colour constancy effects that are non-intuitive: hard to predict.

See my page Colour Constancy Overlays.