Consider that when they realized redshift is proportional to distance in all directions, it either meant we are at the center of the universe, or it's an optical effect. Which was dismissed, as there didn't seem to be any medium slowing the light. So it was decided that space expands!, Because Spacetime!

Totally ignoring the central premise of GR being the speed of light is a constant in any frame. Therefore if space were to expand, the speed of light would have to increase, in order to remain constant.

So two metrics are being derived from the same light. One based on the speed and one based on the spectrum.

If the speed was being used as the numerator, it would be a "tired light" theory, but as an expanding space theory, the speed is still the implicit denominator.

The metric against which this expansion is being determined. The "ruler."

Basically it fails grade school math.

Though multi spectrum "packets" of light will redshift over distance, as the higher frequencies dissipate faster, yet that would mean we are sampling a wave front and the quantification of light is an artifact of its detection. A loading theory of light.

Imagine how the high priests would take to that.