Shaders for weapons trails could be an elegant solution, if you can make one that meets your requirements. It's nice because it happens at runtime, no extra image or animation work. That can matter if you have many weapons. However, it's likely difficult to make it look perfect for all weapons.
Doing it manually is more work, but gives a lot more control. Likely you can find a way to reuse your blurred arc images, possibly using 3-4 for many weapons of various sizes. Just be careful not to use mesh deformation keys for the reasons listed there. You could use bone weights to manipulate the arc sizes. For color, the options you have are just slot color or a shader.
One tip is to use PSD export to export your skeleton in layers, so you can more easily draw the arc. Position the timeline where your character will show the full arc, then do a PSD export of the current frame. This is also a great technique for redrawing attachment images for new perspectives. The exported PSD has all the attachments on separate layers. You can replace one or more layers for the new perspective easily using the rest of the pose as a reference.
