update method
Implementation
void update(double deltaTime) {
  _currentShift += _shiftSpeed * deltaTime;
  if (_currentShift >= _colors.length) {
    _currentShift -= _colors.length;
  }
}
void update(double deltaTime) {
  _currentShift += _shiftSpeed * deltaTime;
  if (_currentShift >= _colors.length) {
    _currentShift -= _colors.length;
  }
}