As stated above, DYHA commonly have the last third of their tail feathers a yellowish green.
Also, feather colors rarely change rapidly, more commonly they fade in color or develop bars either can be caused by stress or diet issues.
General appearance looks good.
If the diet is high in Sunflower seeds they need to be vasty reduced.
Any chance that the bottom of the feathers are becoming lighter in color? This can happen if those feathers are being dragged.