FUV − NUV (left) and FUV − I_E (right) colours as a function of the semi-major axis in kiloparsecs. The empty symbols indicate the values of the central colours calculated when a mask is not applied. The dotted lines connect the central unmasked regions to the successive regions. See Sect. 3.3 for more details. The Perseus ETGs are indicated with solid lines, while the Virgo ETGs from Martocchia et al. (2025) are represented by dashed lines and purple colours. The vertical dash-dotted grey line indicates the projected radius within which a disc of dust is observed in the I_E images of NGC 1270, NGC 1277, NGC 1278, and NGC 1281, see Sect. 4.3.