This paper presents the new G-Fresnel lens-based μ-spectrometer with an image-processing algorithm, such as a color space conversion, in the range of visible light. The proposed μ-spectrometer is developed by using the cost-effective and compact G-Fresnel lens, which diffuses the mixed visible light into the spectrum image, and an image processing algorithm. The RGB color space commonly used in the image signal from CMOS type image sensor is converted into the HSV color space, which is one of the most popular methods to express the color as the numeric value, Hue (H), Saturation (S), and Value (V), using the color space conversion algorithm. Because the HSV color space has the advantages of expressing not only the three primary color of light as the H-value, but also its intensity as the V-value, it was possible to obtain both the wavelength and intensity information of the visible light from its spectrum image. The proposed μ-spectrometer yielded an inverse linear sensitivity (hue vs. the wavelength). We demonstrated the potential of G-Fresnel lens-based μ-spectrometer for the wavelength measurement of visible light such as mechanoluminescence (ML), typically green light across the region of 500 nm.