图表Js:如何根据第三个数据更改点的颜色

const main_data = {
label: '{{ keyword }}',
data: [],
backgroundColor: "#6AC44D",
borderColor: "#1300FF",
pointBorderColor: [],
borderWidth: 1,
pointBackgroundColor : function(context) {
        const response = fetch('/get_json/{{keyword}}');
        const datapoints = response.json();
        datapoints.then(datapoints => {
        const indexed = datapoints.data.map(function(index){
            return index.indexed;
        })                      
        var index = context.dataIndex;
        var value = context.dataset.data[index];
        console.log(value)
        var label = context.labels;
        if (indexed == 0){return value == 'blue'}else{return value =='red'}
    });
}

}

function createChart(){
    async function fetchdata(){
    const url = '/get_json/{{keyword}}';
    const response = await fetch(url);
    const datapoints = await response.json();
    return datapoints;
    };
     
    fetchdata().then(datapoints => {
        const dates = datapoints.data.map(function(index){
            return index.date;
        })
        const positions = datapoints.data.map(function(index){
            return index.position;
        })
        const indexed = datapoints.data.map(function(index){
            return index.indexed;
        })

    myChart.config.data.labels = dates
    myChart.config.data.datasets[0].data = positions
    myChart.update();
    });
}