To leverage hot reloading while developing the actual business logic run elm reactor, then select Colonoscopy.elm from localhost:8000.
The method above doesn't perform the compilation of the .js used in the index.html. If you need to tweak the design or markup, then you'll have to edit the .html directly. This of course doesn't support hot reloading in the browser.
To see changes made to the .elm logic in the index.html page you'll have to rebuild the .js:
elm make Colonoscopy.elm --output=colonoscopy.js