Euphorbia x martinii is a natural hybrid of Euphorbia characias and Euphorbia amygdaloides.The cultivar 'Ascot Rainbow' has lovely gold and blue-green variegated foliage, which sometimes takes on red or pink tonings in winter. In late winter and spring, it has long-lasting heads of red-eyed bracts which are variegated gold and green. It grows to around 90cm and like most of the genus likes a dryish soil in a sunny location.
It looks effective with golden-leaved groundcovers such as Lysimachia nummularia 'Aurea' growing beneath it or nearby the blue flowers of Veronica peduncularis 'Georgia Blue' (usually called 'Oxford Blue') in bloom at the same time. Cut back the flowering stems of Euphorbia when they are spent.
All Euphorbia have an irritant sap in their stems so take care when handling these plants and always wear gloves when pruning. Be very careful not to get any sap in your eyes as this can cause damage to them.
Postscript: note that this and other Euphorbia cultivars are sometimes not long lived in our Sydney climate; mine has since died!