The present work is devoted to the optical properties of Au:TiO2 thin films in order to clarify the role of the Au clusters inclusions in the TiO2 dielectric matrix. Three series of films containing about 30 at.% (29.2), 20 at. % (19.8) and 10 at.% (9.3) Au were deposited by dc reactive magnetron sputtering. On thermal annealing in the range from 300 to 800 °C in protective atmosphere, significant changes on the crystalline phases and clusters dimensions were detected. The most promising optical behavior was found for the film 20% Au:TiO2, where the films revealed some colour changes, evolving from several shades of grey to different tones of red. This change in the optical behaviour of the films was found to be correlated with a cluster size increase from 2 to 17 nm. For higher size values (>20 nm) the films, independently of the Au content, showed a golden appearance colour. The optical changes were confirmed by reflectivity and CIELab colour measurements. Regarding the films with 10 and 30 at.% Au, the results confirmed that there is an evident range of compositions and clusters size where the SPR is more evident. Sample A (10 at.% Au) it seems to be in the lower limit of the SPR showing a typical interferometric behavior on the reflectivity measurements, similar to the TiO2 optical behavior. Regarding the 30 at.% Au one, the results seems to indicate that the amount of gold particles and their grain size is in the upper limit to show a SPR activity.