16進数とMath.random()を理解する助けになったので覚書です。

16進数のカラーコード生成

var hex_numbers =  ["0","1","2","3","4","5","6","7","8","9","A","B","C","D","E","F"];
var hex = "";

for(var i = 0; i < 6; i++) {
  hex += hex_numbers[Math.floor(Math.random() * hex_numbers.length)];
}

var color_code = `#${hex}`;

RGBのカラーコード生成

var rgbArr = [];

for(var i = 0; i < 3; i++) {
  rgbArr.push(Math.floor(Math.random() * 256));
}

var color_code = `rgb(${rgbArr[0]}, ${rgbArr[1]}, ${rgbArr[2]})`

参考サイト

Math.random() - JavaScript | MDN
1. Hex Colors - 15 Beginner JavaScript Projects to Improve Your Front-End Skills! - DEV Community

RGB→16進数、16進数→RGBの変換は下記が参考になりました。

javascript - RGB to hex and hex to RGB - Stack Overflow