One of the most prevalent myths in the planted tank hobby is that dosing more iron fertilizer makes red aquarium plants redder. However, this is also one of the most commonly shared myths in the hobby - as with most myths and legends, it starts with a modicum of truth.
When the aquarium hobby was less sophisticated in the pre-internet era, many planted tanks suffered from overall poor nutrient access. Liquid nutrient fertilizer and aquasoils were not as commonly used back then. In a planted tank with no nutrient supplementation and no soil base, plants could get certain nutrients such as Nitrogen and Phosphates from livestock waste - and Potassium/Magnesium from some tap water, but such tanks were often chronically deficient in iron - as neither tap water nor fish waste provided it in significant amounts. At that time aquarists realized that dosing liquid fertilizers containing Iron improved overall plant health, and the enhancement of red plant coloration was especially apparent. And thus the idea that dosing iron into a tank improved the color of red plants.
Nowadays Iron is common in aquasoil substrates and most liquid fertilizers contain good amounts of it. So for folks that dose their tanks regularly, there is seldom a lack of Iron. It is by far the most common element in aquarium supplements on the market.
For the large majority of folks that already dose a liquid nutrient supplement into their planted tank, adding more iron than what is required does not stimulate the red plant to make more red pigmentation. Iron merely enables the production of pigments; it is not the key factor that stimulates pigment production.
If you are not dosing nutrients regularly, and have no soil base to your aquarium then dosing an iron supplement may well boost plant coloration as well as overall plant health. However, many other factors affect plant coloration besides iron so you need to provide all those other needs as well.
The large majority of aquarium plant pictures on this site were grown in a tank dosed at 0.03ppm Iron per day (using APT complete).
Iron is responsible for a large number of metabolic processes including the formation of chlorophyll and other pigments. In nature, high amounts of it are available in soil. Iron is one of the most common elements present in the earth's crust. However, iron is a micro-nutrient, and plants use a very small amount of it compared to macro nutrients such as Nitrogen or Potassium. An aquatic plant roughly uses 100 times the amount of Nitrogen compared to Iron for growth.
Light intensity (PAR levels) and spectrum plant play a large role in plant pigmentation and visual appeal. Using higher light intensity is akin to giving a plant sun-tanning; color plants often produce protective colored pigments when receiving strong light. Using strong red/blue light can stimulate pigmentation in colored plants and also give red plants a stronger visual color tone.
Many of the aquarium plant pictures on this site were grown using much higher PAR levels than the average planted aquarium. Correcting for underwater factor of 1.3, the stem plants in this arrangement receive 270 - 300+ umols of PAR.
Overall plant health is tremendously important - unhealthy aquatic plants are plants that will not develop good coloration. So providing the whole assortment of nutrients required by plants is necessary, not just iron - this include the different trace elements and other macro nutrients such as Phosphorus/Magnesium etc. Carbon dioxide, principally provided by Carbon dioxide injection, also plays a critical role. You will never find a low tech tank that has as rich colored as a high tech, CO2 injected tank due to the limitations of Carbon dioxide levels.
Rotala 'blood red' in a CO2 injected tank. While this plant can grow well without carbon dioxide injection, its coloration will not be as deep red.
To read more in detail how else to boost red plant coloration, this article goes into deeper detail.
Iron is an immobile nutrient in a plant. This means that plants do not move it easily once it is fixed in place, and thus iron deficiency always shows up in newer growth. For mobile nutrients such as Nitrogen or Potassium, plants can move them from older leaves to newer leaves if supply is insufficient - this leads to deterioration of older growth instead of newer growth.
If you are using an inert substrate, have not been adding any liquid fertiliser (or have only added components, such as NPK, rather than a comprehensive blend) AND observe loss of color in the newest leaves, it is possible that your plants are suffering from iron deficiency. Keep in mind that there are also many other non-nutrient factors can cause a plant to lose color in growth - including adaptation stress of being moved into new surroundings, change/weakening of lighting, or even being shadowed by a more aggressive growing plant by the side.
All of APT fertilisers contain more than enough iron to support even the most fully planted Aquascapes. If you are looking to boost coloration further, look instead at improving light spectrum, increasing CO2 and improving flow.
For some species ( Rotala rotundifolia and its variants colorata/H'ra/etc, Rotala Goias, Hygrophila pinnatifida & H. araguaia, Ludwigia arcuata & L. brevipes etc.) remember that they require low water column nitrate levels to get redder. This is a classic example where adding more nutrition (in this case, nitrogen/Nitrates) will result in the plants becoming more greenish.
To read more on whether nitrate limitation make red plants redder, head here.