Remember, the color of the bubbles only changes for messages YOU send, not messages you receive. Received messages are always black on grey.
What on Earth are you talking about?
As other have mentioned, you can tell if it's iMessage or not in a few different ways, the send button color, the "Text Message" or "iMessage" label interspersed in the conversation, etc, but if you want to talk about the color of the bubbles, that only applies to outgoing messages.