This is a feature request that applies to Acrobat, Illusatrator and other Adobe products. But I post it here, as I believe it is easier to implement in Photshop (maybe).
I'm producing maps for a client, and they are concerned that the combination of colors I have chosen makes it hard for people with red-green colorblindness to read the final product. This is not easy for me, as I have perfect color vision. How am I going to check that my colors are readable to a colorblind person, without having one in my vicinity?
Then I thought that, may be is is possible to emulate how an image looks like for a colorblind person. Then it would be a whole lot easier for me to make good color choices.
I suggest this could be implemented as some kind of colorblind preview mode. It cold also be implemented as a filter layer. To see how it looks like to a colorblind person, I simply add colorblindness-filter on top. This filter could then be used to produce other interesting effects as well. Or, if I mask half of the image, I can produce a demonstration image showing the image half in correct colors, and the other half with "colorblind" colors.
I think that Adobe should seriously look into this topic, as colorblindness is an important aspect of graphic design.
That was already added in Photoshop CS4.