To date the best reds I've seen consistently came from lower lighting and low NO3.
Maintaining low NO3 is easier with low light.
When you add higher light to a higher NO3 tank, what happens?
Less NO3 due to faster growth rates, so the effect si redder plants, but the faster growth rate and uptake of NO3 can cause issues in many tanks and is harder to keep at low levels without more issues, than the low light tank.
You can see this in nature, you can see in the rivers/lakes, wetlands, shaded areas etc.
Here's a Ludwigia repens from about 20 ft down in a very low, but consistent NO3 (0.2ppm)
The light is low, the water filters it and the other plants shade it.
Still pretty red. the plants near the surface in richer conditions: a nice non stressed green color.
Neil Frank's R macrandra is super red at 2w/gal on a 135 gal using standard T12 lights.
He kept 5ppm or less NO3 in the water column and dosed every other day.