• New Document > Enter Sizes > Canvas color = Transparent > OK
• Choose your colour from the Tools > colors panel, and use the rectangle tool to cover the image area.
• In the Layers Panel (F2) select the layer, and reduce the opacity slider to however transparent you want it.
• 'Save As' or 'Export' the image as type 'Fireworks PNG'
• New > Enter Width Height > Background Contents = Transparent > OK
• Choose 'foreground colour' in the tools panel, and use the Paint bucket tool to fill the layer.
• In the Layers Panel (F7) select your layer and slide the Opacity slider down to what ever transparency you want.
• File > 'Save for Web or Devices' > select PNG-24 > Save
Hope that is what you meant!