I was recently contacted by a chap called Ewen MacLaughlin who was writing the Wikipedia entry for the Sheffield Students Union Rag Magazine, The Twikker (which turns out to be the oldest rag mag in the UK; for more information, have a gander at the entry itself).
In 1991, I was invited to provide the cover for The Twikker; something I relished, as it's an old Sheffield institution. Time was tight (I had to bash the cover off between episodes of Timulo and Lazarus Churchyard) so it's not my greatest piece of work, but I'm proud of it as the one occasion I really got to use the knowledge of printing they dinned into me at college.
They'd decided to go all out for a four-colour cover, but the problem with full colour printing (especially in pre-digital days) was the separations - the process of turning a piece of full-colour artwork into four printing plates - which was prohibitively expensive. I managed to create my own separations by drawing the cover in black & white and then making three overlays on tracing paper, one for each of the colours magenta, cyan and yellow. Where you needed a solid colour (like the yellow on the bridge), I'd colour in solid black. Where you needed a tint (like the pale blue of the sky), I'd stick down patches of Letratone dot screen.*
This meant that, when I'd finished, I had a black & white drawing covered by three sheets of paper with black and grey splodges on them, along with the fair hope that when these were assembled by the printer, they'd come out looking like the little hand-coloured rough I'd done to show the Student's Union guys what they should be getting.
And, half to my surprise, it did.
*for you Americans, pretty much the same stuff as Zip-a-tone.