I don't know for sure what Gibson intended. I don't even know if Gibson knows what Gibson intended. But I've always visualized it as black & white snow / static. To me (given my age, etc) that's the iconic view of what a "TV tuned to a dead channel" looks like.

A better example might be https://www.youtube.com/watch?v=h8JZCqaJ068

I always wondered if Mr. Gibson had really thought that part through. Eyes are evolved to detect movement and having the visual motion of TV static overhead from horizon to horizon in cyberspace is a UX nightmare. I doubt anyone could withstand it for long without a stress reaction or some other kind of distress.

The same color as the sky when my redeye coast to coast flight got diverted to Texas at 2am.

...the color of television, tuned to a dead channel.

Duh! ;)