Back in the old days, before desktop publishing revolutionized the look of comics, we had to try all kinds of weird tricks to get interesting special effects. The effect I used a lot was the “surprint” which involved doing a second piece of artwork, to be merged with the original in the film process, which would print in color only.
Here’s a good example from the cover to SWAMP THING #79, which I just unearthed and had a buyer waiting for. The shot of Superman is the black plate (inked by Tom Yeates over my pencils) with the red line indicating where the surprint will go. The second piece of art, with SWAMP THING’s airbrushed face inside tendrils of energy, is the surprint itself, which printed as green on the cover.
And here’s how it looked on the finished cover.