We present an experimental and numerical study of the wavelength dependence, near resonance, of the optical tweezer trap stiffness on three different dye-doped $1\mu \text{m}$ polystyrene spheres with peak absorptions at $\lambda =625$ , 775, and 840 nm. Experimentally, an increase in the trap stiffness of $\sim 35\%$ on the red side of resonance was observed for the dye-doped spheres relative to polystyrene spheres without dye. Numerical simulations for spheres of different sizes, between 20 nm and $1\mu \text{m}$ , and for absorption strengths corresponding to peak extinction coefficient values between 0.0027 and 0.081 were also conducted. Numerical results showed a maximum increase in the trap stiffness of $\sim 35\%$ , which is consistent with experimental results.