Thanks for the suggestions. I don’t think the prime tower is really there for flushing - it is more related to getting a good flow - and when I did my little test without it when going from black to white and back again the colours were ok, But the adhesion wasn’t - and so the part snapped in half. Not a big problem for my print as the two parts can just be slid in.
Personally I like the way that the Bambulab separate out flushing from priming - as when for example I used a Prusa MMU2 - which had the one tower for both, sometimes I found that the tower got quite messy and the prints failed just due to gaps or bulges in the top layer of the tower.
I also agree that you can save a a fair bit of flush waste if you carefully tune the individual swaps - I have done a fair bit of work on this - including writing some macro’s to try and capture the resulting values from the slicer - so that they can be re-applied when project settings or filaments change. But in the end I found the effort required when doing 8-10 colour prints of keeping track of the nearly 100 different combinations of flush lengths too much effort for me. So I have just settled on setting colours as accurately as possible in the slicer, using auto calculate, pushing the multiplier down a bit - to 0.5 to 0.6.
Also I have done quite a bit of work on ‘flush-into’ object selection, flush into infill, and reusing flush and other waste for things like flat sheets - see this thread for more info.
https://forum.bambulab.com/t/making-better-use-of-flush-into-object-in-bambu-studio/29422/72