The damage enemies deal is indeed a range. Before modifiers (such as endless mode), a plain square (of any color) is 1-2, a silver one is 3-5, a gold one is 6-10, and a really big gold one is 11+ . Each enemy has a range of damage, for example a regular fly deals 1-3 damage, so if it shows a silver you know it rolled a 3.
The game used to not even show these dice indicators, imo they made the game a little too easy.