mixure of gaussians for RGB image

Hi, I whould like to isolate license plate of car from an image, the license plate background color is yellow while the digits in the license plate are black so I thought to perform "mixure of gaussians" (on each color channel - three-dimensional Gaussian because I have an RGB image) on the license plate and on the background of the image (not a license plate) and through them calculate for each pixel what is the probability of being from the license plate or from the background. I need help how can I perform the "mixure of gaussians" on RGB image for the background and the license plate(for each color channel - R,G and B)? And how can I calculate for each pixel the probability if he from the background or from the license plate zone? I have attached an image to simulate which images I am using.

Thanks a lot, Itai