Like chipguy said, on scratch you can get opaque - transparent gradients.
On the paint editor its either one or the other, and ghost effect does the whole sprite.
You will just have to have multiple sprites if you want different parts a different transparency.
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no no no no no much easier way to the above: make a circular gradient from yellow to black and then just have a forever go to whatever sprite you want it to go to and forever send to back
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