One of the factors that the production department would take into account when they were choosing the colours for the inside pages was the fact that the paper would yellow. So they would often leave out a yellow plate of colour, because they knew it wasn't needed to get the same effect. I imagine that's part of the reasoning why they only used blue and magenta for the grey in Batman's suit, for many years, as to make grey you would rightly use a yellow plate as well, but they left that out perhaps trusting that the yellowing would make it turn greyer.