A prototype three-frequency (114, 256 and 410 kHz) colour sidescan sonar system built by Kongsberg Underwater Mapping (GeoAcoustics) Ltd was described and preliminary results presented in Tamsett, McIlvenny and King (2016). Recent developments to the prototype colour sonar system are discussed, and new data acquired in a resurvey of the Inner Sound of the Pentland Firth, North Scotland, and in a new survey of an area in the Pentland Firth accommodating four tidal turbines, are described. It is demonstrated that there is considerably greater discrimination between different seabed types in colour sonar images than in multi-frequency data reduced to a greyscale display, or single frequency greyscale images. The advantage of three-frequency colour acoustic imagery over single frequency greyscale acoustic imagery (and multi-frequency data reduced to a greyscale display) is analogous to the advantage of colour television over black-and-white television: a colour image contains three times the information than a corresponding greyscale one. A sidescan sonar system based on pulses of simultaneous ‘pings’ at three discrete frequencies spanning approximately two octaves is a simple approach to multi-frequency colour sidescan sonar. A more sophisticated approach based on ‘chirp’ technology has advantages in principle however there are also disadvantages including engineering difficulties to overcome to achieve a practical system. In the meantime, the salutary ‘keep it simple’ heuristic that applies to three simultaneous pings at discrete frequencies has provided a working colour sidescan sonar system appropriate for use across the 2 environmental sciences and in civil engineering site investigation. In particular the method provides an invaluable tool for acquiring information rich pre-installation baseline data on the nature of the seabed around marine renewable energy installations and for subsequent ongoing monitoring of the condition of the seabed. Tamsett, D., McIlvenny, J. and King, P. The acoustic colour of the seabed and subseabed. In proceedings of Oceanology International 2016 conference, Excel, London, U.K. 15- 17 March 2016.