Your quoted source has that backward -- messages you've received always appear in light grey bubbles, regardless of whether they came in via SMS or iMessage.

It's only messages which you *sent* which appear in green or blue, to let you know whether each message was sent via SMS (which may have incurred a charge from your telco) or iMessage (which is free). At least to a casual glance, there's nothing on received messages to indicate whether they came in via SMS or iMessage.