How to change the color of Exchange's shared calendar in iCal in 10.10?
I too had the same problem with our Office 365 Shared Calendars, but because these aren't user accounts there was no way for the owner to change the colour as there is no owner on these kinds of calendar. I eventually figured out a way of changing the colours which I've created a tutorial for here on my blog: http://matt-stone.co.uk/change-colours-microsoft-office-365-shared-calendars-apple-calendar-os-x/