Colour-magnitude diagram of the adopted reference set of spectrophotometry standard stars. The G magnitudes are corrected for the distance of the stars. The blue symbols show CALSPEC stars, cyan symbols show PVL stars, and red symbols show SPSS stars. M_G is the G magnitude corrected for distance using Gaia DR3 parallaxes (π) with M_G =G+5log(π) − 10.0. No correction for extinction was applied as it is relatively low for all the involved stars (E(B-V)<0.13) and is negligible for this purpose.