Phys­ic­ally based spec­tral ren­der­ing has be­come in­creas­ingly im­port­ant in re­cent years. However, as­set tex­tures in such sys­tems are usu­ally still drawn or ac­quired as RGB tri­s­tim­u­lus val­ues. While a num­ber of RGB to spec­trum up­sampling tech­niques are avail­able, none of them sup­port up­sampling of all col­ours in the full spec­tral locus, as it is in­trins­ic­ally big­ger than the gamut of phys­ic­ally val­id re­flect­ance spec­tra. But with dis­play tech­no­logy mov­ing to in­creas­ingly wider gamuts, the abil­ity to achieve highly sat­ur­ated col­ours be­comes an in­creas­ingly im­port­ant fea­ture. Real ma­ter­i­als usu­ally ex­hib­it smooth re­flect­ance spec­tra, while com­pu­ta­tion­ally gen­er­ated spec­tra be­come more blocky as they rep­res­ent in­creas­ingly bright and sat­ur­ated col­ours. In print me­dia, plastic or tex­tile design, fluor­es­cent dyes are ad­ded to ex­tend the bound­ar­ies of the gamut of re­flect­ance spec­tra. We fol­low the same ap­proach for ren­der­ing: we provide a meth­od which, giv­en an in­put RGB tri­s­tim­u­lus value, auto­mat­ic­ally provides a mix­ture of a reg­u­lar, smooth re­flect­ance spec­trum plus a fluor­es­cent part. For highly sat­ur­ated in­put col­ours, the com­bin­a­tion yields an im­proved re­con­struc­tion com­pared to what would be pos­sible re­ly­ing on a re­flect­ance spec­trum alone. At the core of our tech­nique is a simple para­met­ric spec­tral mod­el for re­flect­ance, ex­cit­a­tion, and emis­sion that al­lows for com­pact stor­age and is com­pat­ible with tex­ture map­ping. The mod­el can then be used as a fluor­es­cent dif­fuse com­pon­ent in an ex­ist­ing more com­plex BRDF mod­el. We also provide im­port­ance sampling routines for prac­tic­al ap­plic­a­tion in a path tracer.