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.
Volker Schöch commented
(Internal reference: RT#18517)