I have written a hex to 8 bit online tool at https://aiseka.com/tool/convert-color-hex-code-to-8-bit

Here is the core code for the tool, code is very easy.

"use client";

import React, { useState } from "react";
import { Button } from "~/components/ui/button";
import { Input } from "~/components/ui/input";

const hexTo8Bit = (hex: string) => {
  const bigint = parseInt(hex.slice(1), 16);
  const r = (bigint >> 16) & 255;
  const g = (bigint >> 8) & 255;
  const b = bigint & 255;
  return `${r},${g},${b}`;
};

const bit8ToHex = (bit8: string) => {
  const [r = 0, g = 0, b = 0] = bit8.split(",").map(Number);
  return `#${((1 << 24) + (r << 16) + (g << 8) + b).toString(16).slice(1).toUpperCase()}`;
};

export const Tool = () => {
  const [hex, setHex] = useState("");
  const [bit8, setBit8] = useState("");

  const handleHexChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    setHex(e.target.value);
  };

  const handleBit8Change = (e: React.ChangeEvent<HTMLInputElement>) => {
    setBit8(e.target.value);
  };

  const handleHexBlur = () => {
    setBit8(hexTo8Bit(hex));
  };

  const handleBit8Blur = () => {
    setHex(bit8ToHex(bit8));
  };

  const handleKeyDown = (e: React.KeyboardEvent<HTMLInputElement>) => {
    if (e.key === "Enter") {
      if (e.currentTarget.name === "hex") {
        setBit8(hexTo8Bit(hex));
      } else if (e.currentTarget.name === "bit8") {
        setHex(bit8ToHex(bit8));
      }
    }
  };

  const handleConvert = () => {
    setBit8(hexTo8Bit(hex));
    setHex(bit8ToHex(bit8));
  };

  return (
    <div className="grid w-full gap-8 rounded-md bg-muted p-4">
      <div className="grid grid-cols-2 gap-4">
        <div className="grid gap-2">
          HEX code:
          <Input
            name="hex"
            value={hex}
            onChange={handleHexChange}
            onBlur={handleHexBlur}
            onKeyDown={handleKeyDown}
          />
        </div>
        <div className="grid gap-2">
          8 Bit:
          <Input
            name="bit8"
            value={bit8}
            onChange={handleBit8Change}
            onBlur={handleBit8Blur}
            onKeyDown={handleKeyDown}
          />
        </div>
      </div>
      <Button className="mx-auto w-1/2" onClick={handleConvert}>
        Convert
      </Button>
    </div>
  );
};