Google Image Search works by analysis of textual clues found within image tags and file names and also in surrounding text on the web page. The search algorithm doesn't have the ability to actually know with certainty what is in an image.
You can see the artifact of close, but not quite right play out in a myriad of examples. In fact, I would venture to say that most image search results are typically more wrong than right. This is because the quality of relevance signals for image subject matter are relatively weak.
So if you do a search for [white couples] and you get a mixture of white and non-white couples, you ought to be happy to have found images that matched your query even if some of the image results did not.
Edit: I just went to Google Image search to double check what you were seeing. I noticed the inter-racial couples you spoke of in your original post. The answer is simple. In many cases, the title of the image was Black and White Couples (or something like that). Your search of white couples is a strong match for half of that title. It is therefore not surprising that these results are showing up in your search for white couples.