Due to their distinctive properties such as high affordability, magnetic properties, and peroxidase mimetic catalytic activity, iron nanoparticles (FeNPs) have drawn great interest in the field of colorimetric sensing. Such nanoparticles show remarkable sensing properties for detecting various analytes such as biomolecules, neurotransmitters, ions, and environmental pollutants. This versatility is highlighted also in this chapter detailing the synthesis, properties and mechanisms of iron-based colorimetric platforms, as well as their subsequent use in the field of medical diagnostic and environmental monitoring. FeNPs, notably Fe3O4, have shown excellent catalytic properties that can imitate enzyme mimetics; these reactions can be employed for oxidation and reduction reactions whose products often produce easily observable colorimetric changes so that they can be used for qualitative and quantitative assessment. In addition, the chapter addresses emerging hybrid systems (e.g., Fe containing Metal–Organic Frameworks (MOFs)), which can facilitate selective recognition of the desired analyte, thereby improving sensors performance. While they do have advantages, issues of stability, specificity, and scalability persist, which has prevented their wider application. This chapter discusses these issues and gives future directions for enhancing the sensitivity, reproducibility and practicability of the iron-based colorimetric sensors.