I would like to add a Terracolor basemap to my QGIS 3 project (http://www.terracolor.net) as an XYZ Tiles connection.
Is there URL service for this?
TerraColor is a paid service provided by Earthstar Geographics LLC. I'm sure their sales people would be happy to answer any questions you have about the types of services they offer. Contact information is available on their website.