As a naturally occurring reducing and oxidizing agent, hydrogen peroxide (HO) has a role in several biotic and abiotic processes. Hence, the onsite, precise, and rapid determination of HO is crucial. Herein, we propose a method for colorimetric detection of HO on the basis of hindered formation of gold/silver core/shell nanoparticles. We used ascorbic acid (AA) as the electron donor to reduce silver ions (Ag) to be shelled around gold nanoparticles and iron(III) edta as an accelerator reactant. Upon reduction of Ag, owing to the formation of core/shell nanoparticles, the color of the system changes from pink to yellow/orange in the spherical nanoparticles and from pink to purple/blue/green/yellow/orange in the nanorods. The nanorods distinguished color in a rainbow manner for higher concentrations of HO, and spherical nanoparticles were critical in the sensitive detection of lower concentrations of HO. HO scavenges AA electrons and therefore inhibits core/shell formation and, consequently, restrains the system's spectral shift and color change. This characteristic was exploited to measure different concentrations of HO. Under well-optimized conditions, various concentrations of HO ranging from 1.0 to 50 µΜ have shown an acceptable linear relationship with different colors and, with a limit of detection (LOD) of 230 nM. Furthermore, various real samples were examined to confirm the practicality of our developed probe.