I am thinking aloud here mainly out of curiosity since I do not have the file you are talking about. Since the PNG has a set opacity all across the image, including the color you are investigating:
- Open the PNG file in Photoshop
- Put a blank layer under it and add some text on this new layer below the PNG layer, the text should be visible
- Target the PNG layer, and press Ctrl-J to create a new copy layer, the text will be less visible now
- Repeat the step #3 until you cannot read the text at the bottom
This should produce the "original" color or something close to it. Now
- Measure the color with the eyedropper and not the hex value
- Put these layers into one group
- Lower its opacity until it matches the original PNG file
You can now read the opacity value. I am adding a few images to show the sequence.
