The link you shared is a private theme link. If you want to keep the theme as private but still share it with people on your project, the best way to do that is to use Libraries collaboration tools. If you don't mind if the link is "public" on the Color site, you will need to publish the theme and then share the published link. See instructions below.
The best way to collaborate on projects is to use shared Libraries. This allows you and your client to share assets (including color themes) and those assets are directly available in your apps. Here's more details on how collaborate with Libraries. Once you have a Library that you are sharing, you can just add the color theme to that library.
Simple Link Sharing
If you don't want to use libraries and would prefer to just provide a link, you will need to make sure the theme is published. And, you need to make sure you are copying the published link to share.
(1) After you create a theme, click the "publish" link.
(2) You will see a "published" notification. Then, you will see a published date on the theme page. Click on the "view" link.
(3) The resulting page is the "public" theme page. You can copy the URL from you browser, or click on the "share" link to copy to clipboard.
Otherwise, you would need
Ahh, I see. Thanks Dave.
Public isn't problematic (facebook, however!), so I've taken your advice.