First of all, you need to use crossed polarizers to get rid of the white specular reflections.
Next, I don't see how "density of yellow" is a texture feature. It's more of a color/intensity/radiometric quantity since it's the wavelength/spectrum of light rather than a spatial variation.
What I'd probably do for a start is to compute two classes: reddish, and yellowish/greenish. Look at the color gamut in HSV color space, looking down along the V axis:
You can see how the yellowish/greenish pixels are separated from the reddish/purplish pixels. I'd calculate the total number of pixels for each color class as well as the area fraction for each color class. I'd also give the mean LAB color of each, and if you have a "true/nominal/reference" color then I'd give the delta E (the "color difference" between the actual object and it's reference color. See my delta E demo in my File Exchange. Also, see my "Color detection by hue" full demo in my File Exchange: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862
