Thanks again!

You know I tried simply making a new sheet in the same workbook ONLINE and copying a few columns from the original over to it.
The behavour did not transfer.

Just to check, I copied the entire year of data into the new sheet and PRESTO, no cyan formatting!

I guess that the online 365 Excel app handled the situation differently to your expereince.
In anycase, the issue is gone. Brilliant.