Ruskin presumably has in mind particularly the frescoes by Giorgione and Titian for the Fondaco dei Tedeschi in Venice, and there does not seem to be any evidence now on which to base a distinction between the contributions of the two painters. Ruskin appears to have derived his knowledge of the frescoes from Zanetti's engraving of 1760 (see Zanetti, Varie Pitture a Fresco de' Principali Maestri Veneziani and Works, 7.297).
Valcanover, 'Frescoes from the Fondaco dei Tedeschi', p. 136, says that the frescoed turrets were removed c.1837 and the building was painted white, 'transforming it into an anonymous building in place of the original, afire with colours, as can be seen in a detail of a painting by the school of Bonifacio Veronese representing Procurator Alvise Grimani offering St. Louis of Toulouse Alms for the Poor (Pinacoteca di Brera, Milan)'.
Another perspective on Ruskin 's account of the state of the frescoes is provided by Wethey, The Paintings of Titian: Complete Edition, III, p. 5, who describes fresco as 'the most fragile of media for the exterior of a building in any climate... At Venice, lying in the sea frescoes were soon dimmed beyond repair'. Giorgione's frescoes are described by Vasari, who also makes the point that 'nothing harms a fresco more than the sirocco winds, especially near the ocean where they always carry a salty moisture with them' ( Vasari, Le Vite, Testo IV.44)
For a discussion of the frescoes, including a list of those still visible in 1936, photographs of buildings and reproductions of engravings see Foscari, Affreschi Esterni a Venezia.