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Here are a few more of my thoughts so far.  1. I wish I had a couple of prisms to play with; we don’t even have a chandelier to rob.  2.  The rate of movement of the red and cyan bands/fringes (not sure which they are yet) in Geoffrey’s video is about the same as the movement of the Sun across narrow slot – 32’arc in 128 seconds of time.  3.  Diagrams of a prism producing spectrums usually start off with a point source of light, but the raw Sun is not a point source of light.  Therefore, when a prism is held in raw sunlight the spectrum appears to be predominantly red to white to cyan, just like the colours in Geoffrey’s video (see youtube video: 'prisms, light, and color' from around 57 seconds in).  4.  Two triangular prisms placed point to point would produce two red-white-cyan spectrums with the reds closest and the cyans furthest apart, so it might be possible to arrange them to move in a colour cancelling manner.  My brain runs out of RAM at this stage. These thoughts don’t solve the problem, but they keep me thinking about colour splitting and merging devices.  DaveP