JSFiddle - React, Tailwind, and code Playground

HTML

<h1>RGB to RGBA converter</h1>
<input id="r_color" placeholder="R"/><br />
<input id="g_color" placeholder="G"/><br />
<input id="b_color" placeholder="B"/><br />
<button type="button">convert!</button>
<br /><br />
	<div id="original">

	</div>
	<br />
	<div id="result">

	</div>

JavaScript

var calculate_rgba = function() {
    var r = parseInt($('#r_color').val());
    var g = parseInt($('#g_color').val());
    var b = parseInt($('#b_color').val());
    
    if(!isNaN(r) && !isNaN(g) && !isNaN(b)) {
        
        var min = Math.min(r, g, b);
        var alpha = (255 - min) / 255;
        
        var r_new = Math.round((r - min) / alpha);
        var g_new = Math.round((g - min) / alpha);
        var b_new = Math.round((b - min) / alpha);
        
        var rgb_string = 'rgb('+r+', '+g+', '+b+')';
        var rgba_string = 'rgba('+r_new+', '+g_new+', '+b_new+', '+alpha+')';
        
        $('#original').html('R: '+r+', G: '+g+', B: '+b+'<br />'+rgb_string).css('background-color', rgb_string);
        $('#result').html('R: '+r_new+', G: '+g_new+', B: '+b_new+', A: '+alpha+'<br />'+rgba_string).css('background-color', rgba_string);
    }
};

$('#r_color, #g_color, #b_color').on('change', calculate_rgba);
$('button').on('click', calculate_rgba);