A fluid to be optically monitored may be optically mixed to modify the apparent color perceived by a colorimetric optical sensor device. The fluid is positioned within a fluid compartment of a vessel also including a contrast compartment adjacent to the fluid compartment. The contrast compartment is configured to have a contrast color with a known main wavelength that is different from the main wavelength of the fluid color. Light from a broadband light source of the colorimetric optical sensor device is reflected off of the vessel, with the received light being received by an optical spectrometer. The optical spectrometer analyzes the received light to determine a main wavelength of the received light, followed by a controller determining a characteristic of the fluid, which is based on the main wavelengths of the received light and the contrast color and the percentage of the vessel occupied by the fluid compartment.
