Doesn't seem like that would quite explain seeing a dark and a light one on the same background as in the screenshot that was included in the quoted post.
that doesn't indicate how long ago it was sent. simply how high or low it is on the screen, as evident by scrolling through messages and seeing their colour change. the date/time on messages doesn't change just by scrolling. there is just a gradient from top to bottom.
the obvious indicator of how long ago they were sent is their position in the conversation and the dates and times attached to them...
the gradient is probably just stylistic, or perhaps to draw more attention to the most recent messages.