Retain RGB values on 0 alpha pixels?

[ReeceGames]

I am making a Minecraft texture pack for bedrock edition, and the grass overlay system puts the overlay color on top of the RGB value of a given pixel, with the transparency of that pixels alpha channel. What this means is, to make a proper texture, you need to put the dirt part of the grass with an alpha of 0.
Unfortunately GIMP replaces the RGB values of a 0 alpha pixel with black.
Does anyone have a way to make a completely transparent pixel that still has a chosen RGB value?