Optical characteristics of porous silicon layers of different thickness acid porosity are simulated and interference spectra are calculated. The results are presented as curves on a colonimetric diagram. For thicknesses up to 500 nm, analysis of colors has shown that the interference color is directly related to the product of thickness and porosity of porous silicon layer. Practical techniques, which enable one to visually define thickness (or porosity) of a porous silicon layer at known porosity (or thickness), are developed, The method has high accuracy and is easy to use, it does not need destruction of the sample and allows one to make the control in selected local areas, A similar technique is useful for in-situ analysis during the porous silicon anodization process. The method applied to dried porous structures and those filled by water allows determination of both thickness and porosity of a porous silicon layer at one time.