Ultramarine Blue and Burnt Sienna mixed together give a great array of greys, browns, indigo and deep blues, like often mentioned in watercolor manuals. I used only these two colors for another Vesuvio portrait. Few weeks ago it snowed on our volcano and, rare event, also on mount Somma, at its left. I swear that these were the exact colors I have seen from my window!
I have an old scanner so I always try to improve the colors with Photoshop. I obtained this second version with the "authomatic color" function. It is more realistic indeed, and I like its nuances, but my original it is more like the first one.