How to modify a selected lines color and transparancy

How do you modify a selected line or object's transparancy, color and linetype? I select it, and from the menus above there is color and line type but nothing happens. And I also cannot see any transparancy modification

What you changed in the top tool bar are the attributes for new entities.
To modify existing entities, use Attributes or Properties tools from Modify menu.
See https://librecad.readthedocs.io/en/latest/ref/tools.html?highlight=modify#modify

Transparency is no common attribute in CAD, at least it is not in LibreCAD, it is not supported.