Interesting that the "NTSC" look you describe is essentially rounded versions of the coefficients quoted in the comment mentioning ppm2pgm. I don't know the lineage of the values you used of course, but I found it interesting nonetheless. I imagine we'll never know, but it would be cool to be able to trace the path that lead to their formula, as well as the path to you arriving at yours
The NTSC color coefficients are the grandfather of all luminance coefficients.
It is necessary that it was precisely defined because of the requirements of backwards-compatible color transmission (YIQ is the common abbreviation for the NTSC color space, I being ~reddish and Q being ~blueish), basically they treated B&W (technically monochrome) pictures like how B&W film and videotubes treated them: great in green, average in red, and poorly in blue.
A bit unrelated: pre-color transition, the makeups used are actually slightly greenish too (which appears nicely in monochrome).
Cool. I could have been clearer in my post; as I understand it actual NTSC circuitry used different coefficients for RGBx and RGBy values, and I didn't take time to look up the official standard. My specific pondering was based on an assumption that neither the ppm2pgm formula nor the parent's "NTSC" formula were exact equivalents to NTSC, and my "ADHD" thoughts wondered about the provenance of how each poster came to use their respective approximations. While I write this, I realize that my actual ponderings are less interesting than the responses generated because of them, so thanks everyone for your insightful responses.
I was actually researching why PAL YUV has the same(-ish) coefficients, while forgetting that PAL is essentially a refinement of the NTSC color standard (PAL stands for phase-alternating line, which solves much of NTSC's color drift issues early in its life).
