In the Mobile Apps class, we do a project like this using SVG and JavaScript. In the class, we completely redraw the scene whenever the user changes one of the sliders. In this version, we keep all the circles but redraw some of them with different opacity and radius. This requires that we create a database of the assigned locations and colors. You can create a new scene using the New Pattern button.
We also do a project like this as a Swift app for iPad. For that app, we don't need sliders because we can just respond to finger drags on the screen anywhere.
In past years, we have put the sliders at the top of the page, but as a class we decided this has some usability problems when running on a touch screen. We moved the sliders below the image to avoid obscuring the scene while manipulating the controls.