I have tried to answer the question from TikZ: changing colour of a path half way along with decorations - unsuccessfully. Maybe it is not possible!?
Can someone tell me why scoped path color change is not implemented?
Scoped path colour is not implemented simply because it cannot be assumed to be implemented in the final document. The lines and so forth in TikZ/PGF are written into the document using the document's method of drawing lines. Since PDF, PostScript, SVG, and so forth have native ways of drawing lines, TikZ/PGF uses those. Since TikZ/PGF tries to be backend-agnostic, it can only truly support those features that are common to all backends. My guess is that for PDF and SVG, it isn't possible to change the colour of a path halfway (pretty sure about SVG, not so sure about PDF, don't know about PostScript). So since the backends can't cope with it, neither can TikZ/PGF.