ID:1466320
 
(See the best response by DarkCampainger.)
Would something like this be possible to do?
Something like what? Create SS13? I would assume so considering it's a BYOND game.
Pomf wants to render the entire screen in greyscale for one player while keeping everything colored for everyone else without having to manually redraw a bunch of icons.

You would have to use icon procs. I don't think you can accomplish this convincingly with just color, alpha, and blend_mode.
Best response
As MisterPerson mentioned, you would have to use icon procs to manipulate them.

Lummox JR wrote an IconProcs library (and accompanying article) that offers some different graphical effects, including greyscale.

Unfortunately, the various icon procs are mostly server-side, so your players would probably have to download the generated icons (some of the icon procs are smart enough to just send the command and let the clients do the work, but I'm not sure if that includes MapColors(), which greyscale would need)
This would need to be a feature request, and you would need to propose a way to handle it in terms of how the user would interact with such a thing.

The best way for this to work, in my mind, would be a client property or client proc. Maybe having access to client.colors or client.graphic_buffer.
The best way for this to work, in my mind, would be a client property or client proc. Maybe having access to client.colors or client.graphic_buffer.

The best way for this to work outright, as far as I'm concerned, would be swapping the entire client over to OpenGL and giving clients and atoms a shader property, that way we can write our own fragment/pixel shaders.

Of course, that's unlikely to happen...
In response to Ter13
Yes, that is an even better suggestion, and ultimately the best way to go if BYOND wanted to be future proof / compatible on more platforms.