Printing a consistent grey: is that still a challenge with comic books such as the Hulk?

In a documentary on the American TV series The Incredible Hulk (1978-1982, dir. K. Johnson, with Messrs. Bixby, Ferrigno etc.), the producer/director explains that he tried to petition Stan Lee to allow him to do a red Hulk because of the association with anger. More to the point, he reveals that Mr Lee explained the reason Hulk was green was because they couldn't do a good grey primer whereas they could do a consistent green. Obviously he's not talking about makeup but the comic book and this is consistent with the wikipedia entry on the Hulk.

Any reason why making a consistent grey would be difficult to do in comics - does that apply to print generally? And is that still a challenge today?