Well, in the Bamberg Apocalypse the description to folio 28v reads "sitting on a golden bow". So in this case it is probably not a rainbow. But for that the mandorla is often similar to a rainbow.
Christ can also sit inside a mandorla on a rainbow:
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Quote:The frame of a mandorla is often graded in color or sculpturally particularly elaborate (diamond rod, perforated rod); it thus recalls the vision of a rainbow in Ez 1:26-28 EU *.
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Christ can also sit inside a mandorla on a rainbow:
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![[+]](https://www.voynich.ninja/images/collapse_collapsed.png)