Ruskin presumably has in mind particularly the work by Giorgione and Titian on the frescoes of Fondaco dei Tedeschi, on which the young Titian appears to have worked as a junior partner rather than a pupil.
The frescoes of the Fondaco dei Tedeschi are described by Vasari in his Life of Giorgione. Ruskin appears to have derived his knowledge of the frescoes from Zanettti's engraving of 1760 (see Works, 7.297). He certainly seems to underestimate the damage caused to frescoes by the Venetian climate.
Valcanover, 'Frescoes from the Fondaco dei Tedeschi', p. 135ff, gives an account of the remnants which have been detached and rescued, and quotes Milesio's description of the frescoes in 1715. Valcanover adds that the frescoed turrets were removed c.1837. The building, he writes, 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 of the sixteenth century by the school of Bonifacio Veronese representing Procurator Alvise Grimani offering St. Louis of Toulouse Alms for the Poor' in the Brera Gallery, Milan ( Valcanover, 'Frescoes from the Fondaco dei Tedeschi', p. 136).
For a discussion of the external frescoes of Venice, including a list of those still visible in 1936, photographs of buildings and reproductions of engravings see Foscari, Affreschi Esterni a Venezia.