Hi,
Why not simply add the name of the color after the img tag in the values of your characteristics ? That way, you don't even need to hover on the images to get the tooltip.
And you can always use such extension
extensions.joomla.org/extension/tooltips
to add tooltip tags around the img tags of your characteristics if you want tooltips on them.
For the stock, it's more complex as it will require customization of the characteristic display area. It would require a developer to work on that for you.