In this work, 3,3′,5,5′-tetramethylbenzidine(II) (TMB²⁺), derived from H₂O₂-horseradish peroxidase (HRP)-3,3′,5,5′-tetramethylbenzidine (H₂O₂-HRP-TMB) reaction system, was used to etch AuNRs to generate different colors of solution. Many enzyme reactions are involved in the production of H₂O₂ (e.g., glucose can react with the dissolved oxygen in the presence of glucose oxidase (GOx) to produce H₂O₂). Given this information, a simple visual biosensor was developed in this study, with glucose as the example target. The detection range of the proposed system varied with the experimental conditions, such as the concentration of GOx and HRP, and enzymatic reaction time. Under the optimized conditions, the longitudinal shift of localized surface plasmon resonances (LSPR) had a linear correlation with the glucose concentration in the range of 0.1~1.0 mM. Meanwhile, the solution displayed a specific color in response to the glucose concentration, thus enabling the visual quantitative detection of glucose at a glance. Compared with the traditional monochromic colorimetry, this multicolor glucose sensor generates various vivid colors, which can be easily distinguished by naked eyes without any sophisticated instrument. Notably, the proposed method has been successfully applied to detect glucose in serum samples with satisfied results.