How to get more Accurate Feathered Edges

[Ofnuts]

If you want sharp and smooth edges use paths:

   

If you just want the same object in another color, either

• Lock the layer alpha-channel and bucket-fill the layer (no selection necessary) with the new color.
  
• Layer ➤ Transparency ➤ Alpha to selection, add a layer, and bucket fill the selection with the new color.