<p>Gold nanostars (GNSs) with many active sites at their sharp edges exhibit outstanding shape-dependent and tunable localized surface plasmon resonance (LSPR) in visible and near-infrared regions, making them an excellent choice for designing visual colorimetric assays. In this work, we utilized GNSs and silver ion (Ag⁺) to design a growth-based multi-color colorimetric method for determining uric acid (UA). GNSs were synthesized by a simple seedless method, employing 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as both a capping and reducing agent. Ag⁺ ions can be reduced to Ag⁰ on the surface of GNSs by UA, leading to a color change of the solution from blue to orange and a shift in the LSPR peak towards lower wavelengths. The extent of this shift (Δλ) is proportional to the concentration of UA. Under optimal conditions, a linear range of 0.1–20 µM and a detection limit of 90 nM were achieved. On the other hand, the observed high resolution multi-color change of solutions inspired us to develop a smartphone-based colorimetric sensor for UA assay. This sensor exhibited a linear range of 0.25–30 µM with a detection limit of 0.24 µM. We applied both methods to measure UA in human plasma and urine samples with satisfactory results.</p>