I think this is an interesting discussion. Since it started, I had a look at some census forms to see how they worded questions. It seems to be a bit different depending on country - I've attached Irish one, but in US they have boxes for Hispanic etc. And all include box for "mixed".
There's a tendency in these questions to use "Ethnicity" rather than "race". There are endless academic arguments as to whether different "races" of humans actually exist, but we know that some medical conditions are more prevalent in people with certain genetic heritage, and that some people are discriminated against because of colour etc, which is why I suppose the questions are asked.
However, I think Jura wanted to know was what sort of person came to mind when the term "mixed-race" was used. As in her family, my mixed-up family has members who are blonde and blue-eyed, light-brown skin and black-haired (probably more typical of what might be perceived as mixed-race), and red-haired with very pale skin. Fortunately, where they live, there is not much obvious discrimination based on skin colour, otherwise the darker ones might suffer
.