Gaia DR3 CAMDs for an illustrative set of young OCs from Hunt & Reffert (2023) with prominent blue main sequences. The vertical axis was scaled to absolute magnitudes using distance modulus values from Hunt & Reffert 2023. Grey points represent apparent G_BP – G_RP colours, while coloured symbols show intrinsic colours on the x-axis, corrected using the colour excesses derived in Sect. 5. The colour scale encodes the Ha EW obtained in this work.