I rarely comment on here so I hope I have responded correctly?

I have a Pi but I have done very little with python.

However, as for a web app solution - although I use C#, PHP (and some Scheme) I would probably be tempted to use web2py if encouraged to use python. Failing that - use something rather popular like Django.

Please note that this below is an example. I have not tested it (or even downloaded web2py) but I hope this serves as an example for what, I believe, you are trying to accomplish. This code is based on general experience and what the web2py tutorials cover.

The only area that needs additonal attention is ensuring the sliders are loaded with the correct values, which I have not done. However, using jquery, I have created an ajax post when the user 'mouse up' on the slider - the values are passed over (which the data variable 'should' be storing in the slider function.

again - expect a few hiccups. Hope this helps.

Controller - controls.py

def slider()

data = request.post_vars

# Do raspberry Pi stuff here

# .. or, if not, send to 'some queue' to be processed elsewhere

View - controls/slider.html

<html>
<head>
    <title>Brightness controls</title>
    <meta charset="utf-8"/>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
</head>
<body>
    <form method="post" action="/controls/slider">
      <input id="brightnessR" type="range" min="0" max="100" step="5" />
      <input id="brightnessG" type="range" min="0" max="100" step="5" />
      <input id="brightnessB" type="range" min="0" max="100" step="5" />
    </form>

    <script type="text/javascript">
     function update() {
         alert("updating");
         $.post('/controller/method', {
             red: $("#brightnessR").val(),
             green: $("#brightnessG").val(),
             blue: $("#brightnessB").val()
         });
     }

     $(document).ready(function() {
         $("#brightnessR").mouseup(update);
         $("#brightnessG").mouseup(update);
         $("#brightnessB").mouseup(update);
     });
    </script>
</body>
</html>