你需要处理有符号整数。请参见如何在JavaScript中将十进制转换为十六进制？

You need to handle signed integers. See How to convert decimal to hex in JavaScript?

console.log(getHexColor(-16731137)); // light blue
console.log(getHexColor(-1218518)); // orange

function getHexColor(number){
    return "#"+((number)>>>0).toString(16).slice(-6);
}