A colour analysis of a carefully selected sample of over 200 Virgo cluster dEs and 8 giant ellipticals ( Es ) in the SDSS yields the following results : a ) the nucleated dEs ( dENs ) follow a tight colour-magnitude relation ( CMR ) which does not broaden significantly at fainter ( M _ { r } \geq - 15 ) magnitudes , b ) the CMR of dENs smoothly transitions to the CMR of Es , but the latter changes its slope at about M _ { r } \approx - 20 , c ) the brighter dENs are consistent with ages of t \geq 5 Gyr and a decrease of metallicity towards fainter magnitudes , but a possible additional luminosity-age relation is not ruled out , d ) it is crucial to treat dS0s separately from dEs since their different colour properties would otherwise bias the comparison of nucleated and non-nucleated dEs ( dEnoNs ) , e ) the dEnoNs show a weak trend towards bluer colours than dENs .