SharePoint Calendar Color Overlay (HEX DISPLAY)
Quick reference below for the 7 Microsoft calendar overlay colors available in SharePoint Online as of 1/19/2020. User Voice community has requested ability to add custom colors, but not yet implemented. Upvote if you like at https://sharepoint.uservoice.com/forums/601165-suggestion-archive/suggestions/11317953-select-own-hex-colors-for-calendar-overlay. Notes on how to create Calendar overlay at https://blog.virtosoftware.com/how-to-color-code-sharepoint-calendars/.
| Microsoft Color | |
| Dark Teal, #00485b | |
| Blue, #0078d4 | |
| Dark Green, #288054 | |
| Olive Green, #767956 | |
| Red, #ed0033 | |
| Dark Purple, #682a7a | |
| Dark Teal, #006984 | |
| Dark Green, #134029 | |
| Brown, #3b3c2a |
Cheers 
Thank you! This helped me make a color key/legend for a calendar overlay.