To be honest - yes I think you are taking the colleagues comment too seriously (I don't think your ideas on not wanting to create opinions based on gender are necessarily wrong though). I believe people ask that question because they want to show an interest, it seems these days there is very little you can say to/ask a pregnant lady - but people ask questions/make statements to acknowledge, I think assuming that she has a preoccupation with nursery color is stretching it a bit.