Remember that this canot show exactly what every protonope or deuteranope sees, but it will show you which colour differences will nearly vanish.
In order to use the tranformation palettes, the image must first be indexed to the web-safe palette. This is not for display purposes, as it becomes rather grainy for a photograph, but just to give the developer or designer an idea of what it will look like on a 256-colour browser, and to prepare it for the next step.
The same steps can be taken with screen dump images of web pages, or any other graphics.
|This is the original image,
scanned in from a photograph
|This is how it looks after applying
the web-safe palette.
Then the image is copied twice, and each copy has one of the transformed palettes applied to it (see Palette files - Using the palettes for instructions).
|This is how the same image looks,
after applying the protan palette
|This is how it looks after
applying the deutan palette.