The floor-tiles and background walls are masked, the "4th colour" (grey, actually ink0) is nearly never used inside those except for the "empty floor" or the "Mouse cursor"...
[[File:Heroquest possible colour variation.png]]
A more clever use of the transparency/mask ink with the Floor/Wall tiles could enable to display this '''background grey''' in those floor/wall tiles but it wasn't done... which is a serious graphical limitation alongside a poor colours choice, which leads to the sprites and scenery being quite blurred into a blue messy hell as the background can't differenciate properly through the use of an additional different colour.
Fire Trap
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