A modified Wiener estimation method is presented for the accurate estimation of diffuse reflectance spectra from RGB values. In this method, the original RGB values are combined with a set of synthetic optical filters to generate another new three color values by using the system matrix. A modified Wiener matrix can then be created with the RGB values and the new color values, which will yield more accurate estimation because of the new color information incorporated. This method is tested on in-vivo color measurements from 200 skin sites in 10 volunteers. The results show that the proposed method improves the accuracy of the estimated diffuse reflectance spectra significantly compared with the traditional Wiener estimation method. Because of the fast computation in Wiener estimation, this method could be potentially developed for a cost effective alternative to a spectral imager.