Phenylketonuria (PKU) is an inborn metabolic disorder, one of the most spread among aminoacidemia, which requires a continuous monitoring of phenylalanine (PHE) level in order to effectively implement the requested dietary regime. Among methods to estimate PHE levels in biological fluids (such as blood and urine) interesting enzymatic-base solutions have been developed, such as ones based on the PHE-Phenylalanine Ammonia-Lyase reaction, which can be used to implement a colorimetric approach. This paper proposes a sensing approach based on the use of a RGB sensor and an embedded architecture to correlate the sensor output to PHE levels. Results obtained demonstrate the suitability of the proposed approach, leading to an accuracy of 0.549 μM in the working range [0-800] µM.