Just wanted to let somebody know, there seems to be a bug with part_color. It seems to ignore the 4th argument (alpha value). I can see from the code how this happens, as line 56 of part_color.cpp is:
pColorD(1.0f, PDLine, m_color1, m_color1, m_color1, m_color2, m_color2, m_color2);
which clearly doesn't make any reference to m_color1 or m_color2 (the alpha values passed into the object.
Apologies if this isn't the correct place to post this, but couldn't find anywhere better given the limited time I have.