You're passing cursorColor as a parameter to _process(). That parameter will override the variable declared above. Normally you don't call _process() yourself. It's called by the engine, and the parameter is a float that represents the elasped time in seconds since the previous call. Since there's no constructor for Color() with a single float parameter, that's why you get that error.
The normal signature for that function is:
func _process(delta: float) -> void:
So you should replace:
func _process(cursorColor):
with:
func _process(delta):
or (better):
#warning-ignore:UNUSED_ARGUMENT
func _process(delta: float) -> void: