'Authentication is required to create a color profile' message when connecting via XDMCP

This message is a native feature of CentOS's Gnome system. This is not an Xmanager setting and instead, you'll need to adjust the relevant Linux setting. Also, simply pressing 'Cancel' will let you proceed with the connection to your remote Linux desktop.

This is a relatively common issue which has been extensively discussed elsewhere. We recommend you refer to the following:

https://unix.stackexchange.com/questions/417906/authentication-is-required-to-create-a-color-profile 

There are several solutions to this issue, but one of the more simpler solution is the use the following:

[root@localhost actions]# cd  /usr/share/polkit-1/actions
[root@localhost actions]# mv org.freedesktop.color.policy org.freedesktop.color.policy.ori