Distribution of F444W magnitudes and F150W−F444W colours of RUBIES targets for the PRISM (top) and G395M (bottom) observations. The symbols and colour scale are the same as in Fig. 4. The colour coding indicates the spectroscopic completeness computed for the RUBIES footprint alone or the full NIRCam area of the parent catalogue. The two sets of panels on the left show all sources, whereas those on the right only show sources with z_phot>3. RUBIES reaches a very high completeness for red sources, especially at z_phot>3. Comparison of the two different measures of completeness shows that RUBIES is biased towards red sources, which is the result of our pointing location optimisation (Sect. 2.5.1). Nevertheless, because the vast majority of sources in the parent sample are faint and blue (Fig. 3), the majority of RUBIES sources are also faint and blue, forming a critical comparison sample to place the rare red sources in the context of the broader galaxy population.