Hi,
Writing variable's value to Excel file in TC and reading this file in your another tool is a valid approach.
However, Excel seems to be an overkill for such task and I would recommend to consider plain text file that you can read from / write to using methods provided by the aqFile object.
Another possible option is to use environment variable, but while TC provides the means to get the value of the environment variable via aqEnvironment object, this object does not have a method to set a value to the environment variable. Thus I would not recommend this approach to you.
> Project.Variables.battery_colour = BuiltIn.ParamStr(<positionOfArgument>).split("=")[1]
This code gets value of command-line parameter and assigns it to the project variable in TestComplete code. You need the opposite - to assign the value of TestComplete variable to something external. Thus this code is not applicable to your task.