Back to Neil's original query, here is an idea for such a colour change indicator based on ammeter current (via voltage drop in the small coil resistance).
A pattern ammeter showed about 6 mohm resistance. Note this circuit is unproven but based on something I did way back when. The 200k value can be varied to suit 6 or 12 V, and transistor gain.
This could be described as a common base comparator, not too far removed from Electronics 101, but perhaps not that obvious. Big plus is that no extra supply required as for the earlier circuit.
Yes Ken, it's that just because you can thing, but a changing colour glow from the ammeter (assuming used to illuminate the instrument) would add deliciously to the paranoia if your charging system is at all dodgy. Fit electronic ignition for best effect
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