Well they aren't leaving you out because of the color of the text bubble. It's because having a phone that isn't an iPhone in the group causes it to fallback to using MMS instead of imessage. They lose a lot of the features that iPhone users love about imessage and the quality of shared images and video is much worse. The moment someone tries to share a video and everyone just gets a blurry smudge of pixels is the moment all the iPhone users get their own group together.