Depending on the amount of pages you have on your website depends on the way in which you should look at generating your meta descriptions.
If possible and ideally every page should have a completely unique page title and description.
Obviously, if your website has thousands of pages and products then you might find that an auto generated title and description is necessary for the "parent" product and then a variation of for the "child" products as you stated above, this should not really be used for a search engine ranking however if they are just product variants you would only SEO the lead or parent product and have the variants removed from the search engine crawls to prevent duplication.
This is how all major e-commerce platforms work and why Magento and even the likes of Amazon operate to avoid duplication of data.
Basically they create a master / lead / parent product and "no follow" all of the child products.
Hope this helps.