This comment is more informative than the entire article.

Does "Paint-by-Numbers" no longer mean indexed colour to the general population?

https://en.wikipedia.org/wiki/Paint_by_number#/media/File:Ка...

It usually does, but the article talks about manually drawing out a grayscale image by following luminance numbers. That’s generally not considered “indexed color” unless you’re using a mapping of pixel values to grayscale values (indexing) to reduce the number of valid pixel values for compression purposes.

This technique is generally known as false color.

https://en.wikipedia.org/wiki/False_color

I mean, I would associate the term pretty strongly with indexed color with irregular regions, not with square pixels.

To me it means I as a bad artist am just filling in colors likely with really bad color mixes on an image. It has a connotation of really low end art.

It’s simpler than that; the pixels were just light intensity values (per the article, they had originally intended to get various shades of grey chalk). The scientists just wound up applying their own “palette” to the intensity values.