Colour-colour relations based on synthetic photometry carried out on the spectra produced for old SSPs with various population synthesis codes (see text for details). (a) Colour transformation from the generic V band of Bessell (1990) to the Euclid system, used for converting the GCLF-properties of Rejkuba (2012). (b) Colour transformation from the g passband of the OmegaCAM instrument on VST (European Southern Observatory), used for comparisons with the FDS survey. (c) Colour transformation from the F850LP passband of the Advanced Camera for Surveys on HST, as used in the ACSFCS survey of the Fornax cluster; F475W photometry differs from the g-band photometry of panel (a) by less than 4% and is not shown. (d) Colours of old single stellar populations in a Euclid colour-colour plane. All passband transmissions used include the filter, the instrument, and the detector.