Process? Create a new layer first? Draw the shape of the white part with pen and ink? And fill it with color.

Then copy the layer.

Hold down ctrl? Click the mouse to copy the layer.

Then the layer will be selected? Then click Select-? Fix it? Expansion? 3 pixels (as appropriate)

Then click Select-Fix it? -Feathers? 3 pixel radius? (as the case may be)

And fill it up? External color (purple in the above picture)

Then move the first floor to this floor first.

it doesn't matter