The authors present a new approach for measuring watermarking robustness in the case of color attacks of rgb-color watermarked images. This approach is applied to evaluate the robustness of our algorithm to watermark an rgb-color image by modifying pixels colors. It is based on Kolmogorov-Smirnov test and it uses fractal dimensions to quantify modified color pixels. The authors test smooth and textured images, smooth and textured marks and 6 types of attacks: JPEG, Median, Noise, PSNR, RML, ROT.