Unfading bloom? In a few weeks nothing’s left for a painter (8)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A MONTH (‘a few weeks’) with the O replaced by ‘a’ plus RA (member of the Royal Academy) (‘nothing’s left for a painter’). The definition suggests an everlasting, but I cannot trace any amaranth so used; however, at least one species does provide a red dye, which might account for the ‘unfading’.