Energy dependence of the millihertz QPO properties in 1A 0535+262 for three epochs (MJD 59169, 59172, and 59181; corresponding to Days 3, 6, and 15). Top panel: Fractional rms amplitude as a function of energy. Bottom panel: Energy-resolved phase lag, Δϕ_QPO, measured with respect to the reference band. Triangles and squares denote HXMT and NICER measurements, respectively. Colours indicate the outburst phase: red (rise), blue (peak), and green (decay). In each energy band, the QPO was modelled with a single Lorentzian component.