LineFormat color gradient COM API
Lines can have color gradients in PowerPoint, but that setting is only available interactively (in the user interface). The LineFormat COM object does not offer this option, and I couldn't find any other COM API for that purpose.
What makes things worse: Lines that have a color gradient do not accept the setting of any new, solid color, i.e., not only is it impossible to set the line color gradient via COM, it is also impossible to get rid of it. But there's more: When trying to set a solid color on a line that has color gradient, the call fails silently. The call is ineffective, but there is no way for me as a programmer to know that it didn't succeed.
Gabriel D'Amico commented
I can offer a solution to this problem since I had the same one and found no viable solution. The only setting that can override the gradient on lines via VBA is the pattern style. You can then set the foreground and background color to the same colour.
Dim oSh As Shape
Dim oSl As Slide
For Each oSl In ActivePresentation.Slides
For Each oSh In oSl.Shapes
If oSh.Line.ForeColor.RGB Then
.Pattern = 1
.ForeColor.RGB = RGB(0, 0, 0)
.BackColor.RGB = RGB(0, 0, 0)
Volker Schöch commented
(Internal reference: RT#18517)