<p>Multimodal image analysis and colourimetry offer valuable tools for measuring and analyzing colour variations associated with rock weathering. As weathering progresses, mineral composition changes, leading to different weathering grades. This study focuses on clastic sedimentary rock slopes and combines the CIELAB colour space with advanced image analysis techniques to develop a quantitative method for assessing weathering. By enhancing traditional visual inspection methods, this approach allows for a more precise and quantitative evaluation of weathering grades. The image analysis by threshold segmentation converts the RGB image to CIELAB to classify the weathering grades. The findings reveal a significant increase in the green–red value a* and blue-yellow value, b* of CIELAB colour space. This increasing value corresponds with a decrease in rock strength, indicating a direct relationship between colorimetric changes and geomechanical properties. The image analysis also highlights the percentage zoning of different weathering grades across the rock surface. The integration of image analysis provides a clear visual representation of the distribution and intensity of weathering, which is crucial for practical applications such as slope stability analysis. Results further reveal the clastic sedimentary rock shows high b* values in areas of high oxidation. The probability of failure is strongly correlated with rock strength and modulus. Low strength and low modulus generally lead to a high probability of all types of failures due to the material’s inability to withstand stress and its tendency to deform. This study successfully demonstrates a new approach to weathering assessment, providing significant improvement over traditional methods.</p>