Rotala's certainly turn red when you let the NO3 get down low and most other nutrients are high. Ludwigia(most) are the same. L granulosus, repens, and arcurata do this.
But folks sometimes take the NO3 limitation too far and kill the plant.
Iron can be some of it, but I can easily turn a number of red plants green by adding nitrogen.
It's not one single element but serveral that get your plants in great shape.
Each nutrient that you are able to get within a good range brings you one step closer to having your goal. It also allows you to have more flexibilty in your routine if all the other nutrients are in good shape. Light, CO2, K, PO4, traces are fairly easy to get in a nice range but folks many times don't go back and make sure these are in good shape.
Many don't know what the PO4 is at all.
Most have CO2 issues(not enough) especially those that use DIY C02.
Many test with poor NO3 test kits.
One thing that works well for me is having a semi low fish load that gets feed regularly, and adding everything except the KNO3 except at 1/2 doses 2x a week.
I get decent results, don't toast the plants, easy routine, does't involve lots of testing to keep things going.
But it depends on your fish load/tap etc. Some tweaking is required.
Bottom line is to keep the NO3 pretty low.
Add KNO3 and you can watch the red turn green in about 1-2 days.
Regards,
Tom Barr