Ever since I began using powdered iron oxide as a media ingredient, I started noticing that many plants, not previously showing much anthocyanin (often misspelled, anthrocyanin) coloring in their leaves, were showing much more noticeable coloration, after being planted in media enhanced with iron oxide.
Several times it was suggested that the iron oxide increased the coloration. Since my usual lighting and general growing conditions already had many of my plants exhibiting nice coloration (to my eye - beauty is in the eye of the beholder), I didn't think much about it.
But, today, with time to spare, I thought to look for any research that might connect the two. I discovered that there was a fair amount of research to indicate that enhancing a plants access to iron and zinc micronutrients, was connected to increased production of anthocyanins.
Below is a photo showing the response I received with Pinguicula gypsicola, right after I first grew out some plantlets in iron oxide enriched media. Another species that shows a strong response is Pinguicula reticulata. Though there may not be a direct correlation, some plants whose leaves were usually a pale green, also became a darker green after the addition of iron oxide.