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?
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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.