os.listdir() returns a list of file names. Thus, filename is a string. You need to open the file before iterating on it, I guess.
Also, be careful with backslashes in strings. They are mostly used for special escape sequences, so you need to escape them by doubling them. You could use the constant os.sep to be more portable, or even use os.path.join() :
folder = os.path.join('C:\\', 'Users', 'Sprinting', 'blue')