We describe a three-step algorithm for the analysis of color images of flames, with the objective of analyzing the combustion process and its control parameters indirectly. The algorithm first extracts the interesting regions, applying a clustering method in the RGB color space, identifying and eliminating irrelevant regions and applying morphological operators. Then it calculates different geometrical parameters for each image. In the final step, it produces the temporal trend and the frequency spectrum for each parameter. The spectra are considered useful for the validation of a theoretical model about the instability phenomenon generated by premixed flames.