First, remove line 11. You can't define functions inside a rule (e.g. Colour --> ...this is the inside part of the rule that I mean), and you don't need to have a function here. (Alternatively, though, you could specify a function that returns a hexadecimal string for the color, and then the rule would call the function inside color).
Second, the color operation takes in a string for a hexadecimal color, so you need to put the string inside quotation marks:
color("#ffffff")
color operation—CGA | ArcGIS
Third, the start rule annotation should have some capital letters: @StartRule
Annotations—CGA | ArcGIS
