Playing from the green when its closed

more right than wrong :thup:

a putting green is a surface prepared for putting

GUR is an area marked, or defined, as GUR by the committee

between these two you get the applicable ruling

if an area of fairway is, for example, being clearly cut to a lower height in preparation for it's role as a putting green then it's a wrong putting green already

if it's marked or defined as a temporary green GUR play prohibited but still cut to fairway height it's GUR

if it's an area of fairway with no preparation, no definition or delineation but you know it's going to be the winter green as usual then then you play it as it lies and suggest to your committee that they do something about it if they don't want people playing from there!