This study examines and evaluates a colourimetric technique that could be implemented as a real-time continuous monitoring system for fungal culture growth in submerged fermentation (SmF) with the presence of solid particles. Fungal SmF of Aspergillus awamori and Aspergillus oryzae was carried out on complex heterogeneous solid media: wheat bran, soybean hull and rapeseed meal, which were constituted of various soluble and insoluble solid particles. As a comparison, another set of experiments in SmF using synthetic medium was carried out. The theory of measuring colour changes in SmF by UV-Vis spectroscopy demonstrates that this colourimetric technique provides valuable information complementary to visual observations. Typical normalized UV absorption spectra were obtained using a scanning spectrophotometer for each colour solution. These demonstrated that samples used in this work have very similar patterns. The typical normalized optical density spectra show a local absorbance maximum near 300 nm, a region where colour absorbance rapidly increases. It was suggested that the colour developed during the progress of fermentation is closely related to either or both of the growth and the actual biomass of the fungus. The findings show that colour-based methods seem to be the most promising approach for biomass estimation in SmF in the presence of solid particles.