We have support for calendar color management in CalDavZAP (also in CardDavMATE and InfCloud). You can check the demo and click the color rectangle (if you click to it while holding the shift button you can also define a HTML hex color). We use the property used by Apple clients, but your server must support that property - otherwise the client cannot store the value on the server (then a default value is calculated from the URL of the calendar using a hash function). Cheers, JM