Despite deploying Fieldmaps to our fieldstaff and hardwiring the portal URL they still commonly try to use the white "Sign in with ArcGIS Online" button instead of the blue "Sign in with ArcGIS Enterprise" button. Many staff even claim the can not see the blue button on the blue background....possibly not the wisest design choice.
The preferred option is to enable an MDM configuration option to entirely remove the 'Sign in with ArcGIS Online' and present only one log in button. And for it to not be blue.
Thanks