J − H vs. H − K_S colour-colour diagram prepared from pre-outburst (2MASS and Pandey et al. 2022) & outburst (SOFI, NOT, IRTF) data of Gaia20bdk. The squares represent zero age main sequence (ZAMS) stars, while the open circles show giant branch (GB) stars (Bessell & Brett 1988). The black line is the locus of unreddened CTTS (Meyer et al. 1997). The two parallel lines formed from pluses represent the reddening path; the step represents the reddening by additional 1 mag in A_V.