Actually, red plants don't need any more Iron than do green plants. This is a myth and is a bit of a red herring.
Fe is an important element in the production of Chlorophyll, which happens to be green and is the main contributor of green color in plants.
Iron is also a very important element in a compound that is directly responsible for the energy production during photosynthesis. The compound is called Ferrodoxin.
These categories of Redox reactions facilitated by Ferrodoxin are present in plants of every color and are fundamental, so they are not more important in red plants than they are in green.
Red colors however, are a result of a variety of pigments including Anthocyanins and Carotenes,
http://onlinelibrary.wiley.com/doi/10.1111/j.1751-1097.1999.tb01944.x/pdf which do not contain Fe, but production of these pigments do require Iron as well as PO4 and other nutrients.
The production of these pigments is extremely expensive, so it is not a given that leaves will automatically turn red just because of one specific factor. The plant must have a really good reason to produce them.
The pigments develop in response to stress due to excessive light, which typically causes a fault in the tissues which results in the unintended creation of Reactive Oxygen Species (ROS) such as Hydrogen Peroxide.
Anthocyanins and other red pigment combat the ROS by either reflecting the wavelengths of light that may cause the stress or by neutralizing the ROS. In effect, the pigments are Anti-oxidants.
That's why it is claimed that eating red fruits and veggies, such as pomegranate, blueberry and so forth are good for you.
So there are a variety of reasons that red plants can turn red as well as reasons why plants that are supposed to be red do not turn red, not the least of which include genetics.
As always, the building blocks of all pigments and of plants health in general should be made available.
Plants lacking in Fe will be pale regardless of what color they are supposed to be, but it is unnecessary to dose more Iron for red than for green.
More important is the issue of Iron availability due to poor chelation and the ability or inability for the plant roots to absorb the fallen Iron.
Fe+++ can be easily reduced to Fe++ in the tank once the chelation is lost and then it becomes less soluble in the substrate depending on pH.
Different plants have different talents in absorbing Fe once it is no longer available in the water column and oftentimes it is necessary to use a more effective chelator to keep the Iron in the water column and to keep it chelated. Higher water GH makes it more difficult to keep the Iron and other metals cheleated.
Poor CO2, poor PO4 and poor flow/distribution can also result is a plants inability to express it's colors, primarily due to poor health.
Also don't forget that you can also add more red lighting to the tank and that will make red plants look more red.
Cheers,