Check out the FAQ on this page, there is the path on which RenPy is checking for mod files.
I used a tool like cx file explorer the last time I moved the mod to the android installation. Or if that's possible for you, plug your phone to a PC, navigate to the respective folder and put the walkthrough mod there, same for the cx file explorer.
There is a default red color already available, if you want another red, try this value:
#FF0000
Don't forget the hashtag, it's necessary.