Give your calendar its own colours back, colour copies by where they came from, and fix Outlook copies that arrive grey.
Under Event color in the sync's settings, pick the crossed-out swatch — the one tooltipped "No color (use the calendar's own color)". Copies then inherit whatever colour the destination calendar already uses, exactly like events you create there yourself.
It's in the sync's Appearance settings, below the title options.
Picking a colour on a sync is useful when copies land in a calendar you also use for other things — the colour is what tells you "this one came from somewhere else". But if you keep every calendar its own colour and rely on that to read your week, a sync colour overrides it and your view goes flat. The crossed-out swatch is what gives your calendar's own colour back.
Existing copies keep their old colour until the sync runs again.
On Pro you can colour copies by their source: everything from your work calendar in one colour, everything from a client's in another, no matter which calendar they land in. In Appearance, above the colour swatches, switch from One color for everyone to Match source calendar, then set a colour per source calendar.
Colour is the only thing the source layer controls. Every other setting is decided by the calendar events land in.
This one isn't a sync problem, and the fix is on your side.
Outlook has no per-event colour. It has categories, so a colour on a synced copy becomes one of Outlook's six built-in category names, and Outlook draws whatever colour that category is set to in your mailbox. If you renamed or deleted the default categories (plenty of people do), the category name on the copy matches nothing in your mailbox. Outlook on the web falls back to grey; Outlook desktop ignores it and uses the calendar colour.
To fix it, create a category with that exact name in Outlook (right-click an event → Categorize → All Categories → New) and give it any colour you like. Every copy carrying that name picks it up.
We can't create the category for you. Doing it would need permission to change your mailbox settings, which is a much broader permission than calendar access, and asking for it to fix a colour isn't a trade we're willing to make.
Verified against the product on .
Still stuck? Support will read your actual setup and tell you what it sees — describe what you expected and what you got.