Clinicians need a way to rapidly and reliably test the correct functioning of near-infrared spectroscopy (NIRS)-based oximeters. Therefore, optical phantoms for quality assessment of NIRS oximeters are needed. The fabrication of such phantoms that mimic the optical properties of biological tissue in the NIR range represents a challenge. To enable their development, the aim was to characterize the absorption and scattering spectra of different dyes. The optical properties of silicone SILPURAN 2420 with 11 color pastes of type ELASTOSIL were measured in the 500 to 1000 nm range by a spectrometer with an integrating sphere. In addition, two commercial frequency-domain NIRS devices, the ISS OxiplexTS and the ISS Imagent, were used to assess the optical properties at specific wavelengths. The evaluated colors present mostly features in the visible range below 650 nm, but two colors include peaks in the near-infrared region, simulating low tissue oxygenation values. These colors were used to create an optical phantom, which matched the designed StO value within an error of only 4%. This set of dyes already enables simulating many different spectra, thus achieving a first step on the way to a long-term stable comparison and validation method.