When I Paintbucket fill a new layer with Red (or other color) at zero Alpha - Transparency in the Color window, it Color Picks as black with zero Alpha - Transparency.
In Overwrite mode.
This, even before saving.
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If I make a zero opacity red (or other color) layer as described in post #5,
saving as Optimized PNG, it produces a zero opacity layer with color values preserved.
This is based on my actually testing this, not on my belief.
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Color Picking the attached OptiPNG will show different colors at different locations, all zero opacity.
AlphaColorOptiPNG01.zip