Written by COLOUR_STRIP when a building's colour, which runs from a wall's foot up to the top of the play area, is filled in a row's last byte: the attribute rows count up the screen, so the top row's last byte is the buffer's last, and the fill goes one past it (measured: 28 of 752 colour fills in a run reached it, all at the right-hand edge; none started below the buffer). Nothing reads it, and the buffer's own fill (SHOW_PLAY_AREA) starts below it.