Building integrated photovoltaics (BIPV) offers efficient utilisation of land area and local renewable energy generation. Attractive aesthetic aspects and efficient energy generation are important drivers for market acceptance of solar panels as building elements. Coloured modules improve aesthetic aspects at the expense of efficiency loss and increased cost. For a given lightness and colour, a minimum energy loss due to reflected photons is unavoidable. Additional efficiency loss may come from parasitic absorption in the colour providing layers and from unwanted reflection and absorption in the nonvisible near infrared wavelength range. In this paper we present a novel method to identify the different loss contributions: visible reflection loss, near infrared reflection loss, and parasitic absorption loss. The methodology is applied to silicon solar cells where colouration is introduced by interference from a thin indium tin oxide layer on top of the SiNx:H antireflection coating. The method presented here will be a useful tool to optimize efficient colouring of solar cells.