Honestly, I have difficulties following your steps. Alas, the LineColor is the global setting. If you change that, all connectors will take that color - except: a diagram uses Color override which is stored in t_diagramlinks.
q.
Sorry about that, my bad. But your response may have the basis of, at least, explaining the behaviour.
With vertices, setting the line color to default (whether at the t_object or t_diagramobject level) will set the line to the colour shown in the MDG definition. So for example, if I make my Classes blue and my Activities red, setting each to -1 will generate the appropriate border colours on the diagram. Actually some testing reveals some EAUI. If the BorderColor value in the MDG is NOT 0 (or -1) then the birder color will be set correctly (regardless of the Line color setting in the [Ctrl+F9] dialog). If it is -1 then it is set to the value in the [Ctrl+F9] dialog. If it is 0, then it is NOT set to BLACK, but to the default value!
With arcs, that doesn't happen. The BorderColour setting in the MDG doesn't seem to have ANY effect. The default colour is ALWAYS the value of the Connector Line Colour in the [Ctrl+F9] dialog.
For consistency (and to provide the ability to control the diagram rendering), there should be a hierarchy:
Project default, Metatype (MDG or Local) Default, Element Default
The colour value to be rendered could then be CONSISTENTLY determined: Check element, check metatype (from MDG or locally defined), check project (in that order).
Thoughts?
Paolo
[Edit: There is NO project default - it's an instance Default. EACH EA instance sets it's own set of colour defaults that it applies across all the repositories that it opens.]