High Accuracy vs Low Time Investment
Generally there is no single correct approach to handling multiple variations for your PPC campaigns - it all depends on how much time a seller is willing to invest.
For the highest accuracy possible in your adgroups' reportings, the general notion is to only include different products in the same adgroup if the keywords within that adgroup are all relevant for each product.
Also, each keyword should be placed into one adgroup only to avoid mixed up reportings.
High Accuracy Approach
If time is no issue, one possibe adgroups structure could be the following:
Adgroup 1: Include all products + all generic keywords
Generic keywords would include the keywords that fit to every single variation - ie. keywords that do not include any variation specific attributes such as colors or size.
Example: A sport t-shirt for men with different colors would have all its variations in Adgroup 1, and the keywords would include "sport t-shirt", "sports shirt" , "men's sports shirt".
Adgroup 2 - X: One product for all variation specific keywords
For all other keywords which, for example, inlcude a certain color, create one adgroup for that one specific variation.
Example: Adgroup 2 would include only the blue, large sports t-shirt variation, and thus only include keywords such as "sports shirt blue", blue large shirt", "large sports shirt men".
Low Accuracy Approach
In order to keep it simple, add all variations into one adgroup with all keywords.
This will, however, lead to keywords working well for certain keywords in that adgroup and badly for others, which makes the reporting and thus optimization more difficult.
Additionally, if a variation is out of stock, you would still have active ads for the that varation's specific keywords.
Webinar on Campaign Structures
For general tips on how to setup your PPC campaigns, check out this webinar recording:
In the 23rd minute our CEO Franz also adresses the topic of multiple variations.
Alternatively, check out this blog article on optimal campaign structures.