I have had this same problem, but what has really annoyed me is that one person, who I email regularly every day, was initially assigned a green circle for the last year.
For some reason in the last 24hours, my circle colour (which was always black), and their circle colour (green), has changed to an IDENTICAL navy blue. Visually, this is extremely irksome - and makes it difficult at times to tell the emails apart! I can't
find any assistance on how to change it.
Given outlook changed the colours for whatever mysterious reason, surely the colours can be changed back, or just changed in general.
I hope this is resolved soon, because whilst I can accept there may not be a feature to choose colours, I am absolutely furious about the change without notification or explanation. At first glance it now looks like I am writing an email chain with myself.
Please let me know if I've failed a basic IQ test, or fix the problem.