JSFiddle - React, Tailwind, and code Playground

by Blender

HTML

<textarea id="colors" rows="10" cols="10"></textarea>
<textarea id="hex" rows="10" cols="10"></textarea>
<input type="button" value="Go!" id="go" />

JavaScript

function colorToHex(color) {
    if (color.substr(0, 1) === '#') {
        return color;
    }
    var digits = /(.*?)rgb\((\d+), (\d+), (\d+)\)/.exec(color);

    var red = parseInt(digits[2]);
    var green = parseInt(digits[3]);
    var blue = parseInt(digits[4]);

    var rgb = blue | (green << 8) | (red << 16);
    return digits[1] + '#' + rgb.toString(16);
};

document.getElementById('go').onclick = function() {
    var colors = [];
    
    colors = document.getElementById('colors').value.split('\n');
    
    for (var i = 0; i < colors.length; i++) {
        document.getElementById('hex').value += ('\n' + colorToHex(colors[i])).toUpperCase();
    }
};