Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You wrote a call with several things to write, and a colour setting that comes before all of them. And now the engine cannot tell who the colour belongs to. A parameter belongs to the being it follows. Look at the example: write, then the colour red, then a, then b. Is red for a? For b? For both? There is no honest way to choose, and the engine refuses to guess. If there were just one being, the order would not matter, because there is only one place the parameter could go. It is only the several-beings case that raises the question. The fix is easy: put the parameter after the being it describes. Write a, then red, then b, and red belongs to a, while b stays plain. Or, for even more clarity, give each being its own bracket group. The message names the parameter and the row. Say who owns what, and the call reads clearly to both the engine and a human.
Text from the Rush engine's error catalogue.