How do I merge a spot channel into rgb or cmyk while preserving the color?

c.pfaffenbichler​ Although we did a fair amount of six-color work at that time, we used spot colors in the image -- which, in this area, were referred to as "touch plates" or "kickers" created from modified versions of the separations in the four-color set. I agree, budget constraints were a limiting factor because in addition to prep and press in those pre-Photoshop days "proof" often meant "press proof", too.

Derek, the orange and green you referred to reminded me of years gone by when Pantone came up with Hexichrome, a six-color process system that added those to colors to the CMYK set.  I just checked Wikipedia and, sure enough, it showed up there and I thought it would interest you and some of our cllreagues. To quote from the site:

"Hexachrome is a discontinued six-color printing process designed by Pantone Inc. In addition to custom CMYK inks, Hexachrome uses orange and green inks to expand the color gamut for better color reproduction. It is therefore also known as a CMYKOG process.

Hexachrome was discontinued by Pantone in 2008 when Adobe Systems stopped supporting the HexWare plugin software. While the details of Hexachrome were not secret, its use was limited by trademark and patent to those obtaining a license from Pantone. The inventor of Hexachrome is Richard Herbert, who is also the president of Pantone Inc.^[1]"...​

I was still actively involved with offset litho at that time and as I recall, the G in the set ran at the M angle (where there would be no M) and the O was placed at the C halftone angle (where there would  be no C) to deal with moire potential. So much for the old days.