Technically, the Carbon-Carbon bonds signfied in the diagrams by the straight lines between points without a designator are the glue. However, Magnesium is essential to Chlorophyll simply because it's the central unit of the molecule. Interestingly, that large structure is a substituent of a very long hydrocarbon chain which can be seen in the bottom left (H39C20) making Chlorophyll a lot larger molecule than most of us may think.
Looking at the middle MN4 group it's pretty easy to see why our plants go green when they get Nitrogen or Magnesium deficiencies. Notice there's no Iron or Potassium in the molecule.
One of my hypotheses on red coloration and Nitrogen deficiencies is that the plants have the red pigments already but they're covered up by the Chlorophylls. When the plant is N deficient and has to resort to breaking down Chlorophyll to get at the Nitrogen it needs the red, orange, and yellow pigments come out from hiding and can be seen. Since these Carotenoid pigments are less effective than Chlorophyll higher light is needed to obtain the same amount of sugars, hence red plants needing higher light.
It's worth a try to find out. I actually had the idea at the CAPE meeting at Neil's house. His plants were the brightest red I've ever seen and he said that he doesn't fertilize much of anything. I wouldn't do it with your prized species, but Ludwigia repens would be a really good test subject because of its green and red characteristics. Just be careful, they still need these chemicals for other things too. Don't totally deprive them of N and Mg. Actually, you've on well water, aren't you? It's going to be really hard to not give them enough Mg.
I probably wouldn't want to do a real test as I absolutely hate testing So, I rarely do much of it. I don't fert much either, although I do from time to time use some epsom salts and calcium carbonate, and Kent Bontanic Grow, because my well water is extremely soft (1kh, 1gh). I do have Equilibrum, but have only used it once. I use the normal Flourish for my 55 but not my shrimp tank because of the copper.
My absolute easiest tank, however, is my 29 gallon shrimp tank, and I'm actually less happy with it since I raised the kh to 3. I'm letting it go back down as my Toninas are much happy with the lower kh.
However, I will dose some more epsom salts now just to see if it will increase reds as an informal kind of experiment