A seminar about women in STEM fields I'm attending a talk that will be given by a professor that runs a social-cognitive development lab at our school, and the topic will be about his work that attempts to explain why women are less represented in STEM fields and how early in life this occurs. 

I suspect that plenty of STEM faculty and students will attend; moreover, I expect there to be uncomfortable topics regarding gender and race.  

To what extent should I participate in question and answer with the speaker, if I see my own colleagues in the audience?  I would like to participate actively if I feel I have something interesting to add but that could necessarily be a question about gender and race.
I have been to seminars like that, including ones where large numbers of my colleagues are present.

So long as you have meaningful and productive questions (i.e. don't ask a question for the sake of asking one) and ask them as respectfully as you would any other topic, there isn't a problem.

I will note that choosing not to attend is also sending a message, and in the seminar I was in, a number of colleagues expressed disappointment that the room was nearly entirely women (in this case it was about the status of women in the field).