JSFiddle - React, Tailwind, and code Playground

HTML

<span class="swatch" style="background-color: #d1f2a5;"><strong>paste here</strong><em></em></span>
<span class="swatch" style="background-color: #aba9ff;"><strong>lalala</strong><em></em></span>
<span class="swatch" style="background-color: #abcdef;"><strong></strong><em></em></span>
<span class="swatch" style="background-color: #f00ba4;"><strong>foobar</strong><em></em></span>

JavaScript

$('span.swatch').each(function(i,e){
    bgcolor = rgbToHex($(e).css('background-color'));
    $('> strong', e).html(bgcolor);
});

function rgbToHex(rgbString) {
    var parts = rgbString.match(/^rgb\((\d+),\s*(\d+),\s*(\d+)\)$/);

    delete (parts[0]);
    for (var i = 1; i <= 3; ++i) {
        parts[i] = parseInt(parts[i]).toString(16);
        if (parts[i].length == 1) parts[i] = '0' + parts[i];
    } 
    var hexString ='#'+parts.join('').toUpperCase(); // "#0070FF"
    return hexString;
}