The colours of Microsoft's logo

I was wondering, if the blue in Microsoft means the Windows division, as in, mobile and PC, and green is for Xbox, as in media services like music etc, and red is for Office, what does the yellow stands for?

If Bing services like Bing News, Bing Maps etc. are the meaning of the yellow bit, what category of service is it? Basic tools are something? I dont think it makes sense.. Can someone explain?