function getAutoColor(color, opt) {
    return color !== 'auto' ? color : (opt && opt.autoColor) ? opt.autoColor : null;
}
Consider the case: in a scatter,
label: {
    normal: {position: 'inside'},
    emphasis: {position: 'top'}
}
In the normal state, the `textFill` will be set as '#fff' for pretty view (see
`applyDefaultTextStyle`), but when switching to emphasis state, the `textFill`
should be retured to 'autoColor', but not keep '#fff'.