oh.. my experience

You definately can not wear bright colors. I have seen one or two wear dark red in the funeral. It is not appropriate. You should wear black dress or formal black suit. Your boyfriend should wear black suit with black tie. When you meet the person whose mum passed you should say I am sorry to hear that... When you meet other teamates of your boyfriend's you should say nice to meet you. You can smile a bit but not laugh. Stay quiet and talk quiet overall. There is nothing to worry about. Normally there is a viewing of the dead and then the family member and friends say some final words. You are just there to listen. There is really not much time for your to socialize with other people.