Default node colors should be a stable random color

Each node type should start as a random color that is stable based on on the node type string.

* Ensure colors are in a range that is visible with the background
* Ensure user can still customize the color

Locate the code that initializes default colors for node types and the path used when users customize those colors. Verify that the same node type receives the same visible color across sessions, that colors contrast with the background, and that user-provided colors still override the defaults.

Written by the indexing model from the issue text.