The decision of the architects to develop the theme of an Oriental walled city, and the natural setting of the site, Mediterranean in its sea and sky, led Guerin to select Oriental colors. Aiming at simplicity, he decreed that not more than eight or nine colors should be found upon the subdued palette from which he would paint the Exposition. Then he took into consideration the climate and atmospheric conditions peculiar to San Francisco. Every phase of sky and sea and land, every shadow upon the Marin hills, across the bay, was noted in choosing an imitation of natural travertine for the key color of the Palaces. This is a pale pinkish-gray-buff, which may be called old ivory. It is not garish, as a dead white would be, especially in the strong California sunlight, but soft and restful to the eye. It harmonizes with the other colors selected, and, most important of all, it avoids a certain "new" effect which pure white would give, and which is deadly to art. Paul Deniville, who had already developed a successful imitation of travertine, was engaged to make the composition to be applied over the exterior walls. This is a reproduction in stucco of the travertine marble of the Roman palaces of the period of Augustus. This marble is a calcareous formation deposited from the waters of hot springs, usually in volcanic regions, and is common in the hills about Rome. It often contains the moulds left by leaves and other materials incorporated in the deposit. These account for the corrugations of the stone when it is cut. In California, as in other regions where hot springs are found, travertine is not uncommon. It is found notably in the volcanic district of Mono County, and elsewhere, sometimes in the form of Mexican onyx, which is only a translucent variety of the same marble. In its reproduction here the marble has been imitated even to the natural imperfections which roughened the Italian stone. In the concave surfaces of the ornamentation the color has been deepened, so that it appears sometimes as a rich reddish brown. All this enhances the antique effect, making the palace walls and columns still more like those of the old Roman construction. Besides the travertine the eight other colors employed are: 1. French Green, used in all lattices, flower tubs, curbing of great plats, where it complements the green of the grass, In the exterior woodwork and some of the smaller doors. 2. Oxidized Copper Green, a peculiar mottled light green. All the domes, except the six yellow ones in the Court of the Universe, are of this light green. It forms a sharp contrast with the blue sky and a pleasing topping to the travertine walls. 3. Blue Green, found in the ornamentation of the travertine, and in the darker shades at the bases of the flag poles. These first three colors, all in tones of green, are regarded as one unit in the spectrum of nine colors allowed by Guerin.