How do I create a module that uses composer to manage a 3rd party library?
In order to have composer download a 3rd party library, you need (as you have already done) a composer.json file within your module. This file should contain the required 3rd party libraries. There are however quite strict guidelines for creating a proper composer.json file for Drupal in which you should at least define the following:

{
    "name": "drupal/example",
    "description": "This is an example composer.json for example module.",
    "type": "drupal-module",
    "license": "GPL-2.0+"
}


Running composer from the module root is indeed not recommended as this might become quite time consuming when installing many modules, not to mention updating all of them once in a while.

Luckily the Drupal guys have done some work, allowing composer to manage all Drupal dependencies, which is out of beta since a few weeks.

This allows you to use composer from the Drupal root to require modules as well. During the downloading of the module all dependenies of it will be resolved as well.

On a small sidenote;
Reading through that blog post again leads me to assume the following should also be included in your module's composer.json file (as of writing the first link also shows an outdated message):

    "repositories": { 
        "drupal": {
            "type": "composer",
            "url": "https://packages.drupal.org/8" 
        }
    }


As for your situation, it seems you should commit a file similar to this to Drupal.org:

{
    "name": "drupal/jquery_colorpicker",
    "description": "jquery_colorpicker module.",
    "type": "drupal-module",
    "license": "GPL-2.0+",
    "repositories": { 
        "drupal": {
            "type": "composer",
            "url": "https://packages.drupal.org/8" 
        }
    },
    "require": {
        ...
    }
}


And then run composer require drupal/jquery_colorpicker from your Drupal root.

UPDATE:
After reading that blog post for the fourth time, I came to conclude that the repositories part is with high probability only required for the root composer file and thus not required for individual modules.