ABSTRACT The signal from a shock breakout (SBO) is the first signature of a supernova explosion, apart from gravitational waves and neutrinos. Observational properties of SBOs, such as bolometric luminosity and colour temperature, are connected with the parameters of the supernova progenitor and explosion. The detection of SBOs or the cooling of SBOs will constrain the progenitor and explosion models of collapsing stars. Since the recent launch of the eROSITA on the SPECTRUM-RG spacecraft, the detection rate for SBOs is a few events per year. In the current study, we examine the analytical formulae derived by Shussman, Waldman & Nakar (arXiv:1610.05323). We use four red supergiant models from their study, while running explosions with the radiation hydrodynamics code stella. We conclude that there is a good agreement between analytical and numerical approaches for bolometric luminosity and colour temperature during SBOs. The analytical formulae for the SBO signal based on the global supernova parameters can be used instead of running time-consuming numerical simulations. We define the spectral range in which analytical formulae for SBO spectra are valid. We provide an improved analytical expression for the SBO spectral energy distribution. We confirm that the colour temperature is dependent on radius derived by analytical studies and we suggest using early time observations to confine the progenitor radius. Additionally, we show the prediction for the SBO signal from red supergiants as seen by eROSITA.