<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def kind(self):
 """ What color mode has been set. Returns mode: 'face', 'vertex', or None """

        self._verify_crc()
        if 'vertex_colors' in self._data:
            mode = 'vertex'
        elif 'face_colors' in self._data:
            mode = 'face'
        else:
            mode = None

        return mode