How to change the color of Exchange's shared calendar in iCal in 10.10?

I've been in trouble for some days with the Exchange services on our Apple ecosystem. Right now, I want to change the color of the shared exchange calendars. Changing the color of the local calendar works, same for iCloud ones, and same for my own calendar. See screenshot below. System is in french as you can see…

BUT it doesnt work the same for the shared calendar. Here is when I right clic the calendar :

FYI when I right clic the name of the person above the calendar (on the screenshot it's behind the red rectangle) it just says something like (sorry for the translation) : open the delegate in a new window.

And then, once I clicked "read the informations", this is what I get. A great greyed selector :

Of course when I click the color selector nothing happens. And when I click the "informations on the account" I get the account window, the one you get when you click in the menu on calendar > accounts.

Well, does anyone have an idea to change these damn colors ? I googled the thing but didn't find any consistent thing.

PS: there is a quite similar topic here on StackExchange but it doesn work for me since every shared calendar have the same name (and as you saw the calendar name is greyed too so it can't be changed).

THANKS !!

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/

I followed the instructions from Matt Stone http://matt-stone.co.uk/change-colours-microsoft-office-365-shared-calendars-apple-calendar-os-x/ and I can confirm it worked for Mojave 10.14.6. I will update if it reverts overwriting the plist change.

An important part is making sure there are no active CalendarAgent processes. I might add a step of ps ux|grep [C]alendarAgent to check for them- and if still active, quitting more applications and re-doing the killall.

In case that page goes away: a command-line version of the instructions:

1. cd Library/Calendars
2. grep -ir "human-readable unique name of calendar"
3. search results will have a common directory pattern at the start.
4. cd that directory
5. there will be a subdirectory ending in ".exchange"
6. cd that directory.
7. less Info.plist - look for <key>Color</key> and the next line has a hex color. That color should match the old color as appeared in ical.
8. killall -u yourusername CalendarAgent
9. wait 30 seconds
10. ps ux |grep [C]alendarAgent
11. if anything is returned, you're still running something that restarted calendaragent. quit more things. Go back to 8.
12. rm ~/Library/Calendars/Calendar\ Cache-bak
13. vim Info.plist and replace the color with your preferred color.
14. re-start the calendar app. et voila!