Yes that works and your initial color will be 0,0,0,0 in the editor.... but...
If you do :
--Color
Script.Cool = Vec4(1,1,1,1)--Color
--Vec3
Script.Vec3Test = Vec3(1,0,0)--Vec3
Throws:
...: Attempt to call a non-callable object
Or if you try getting the value from your first example
SetColor(Vec4(Cool)) -- will result in same error