Blue is the least efficient so I would guess red or green.
It isn't about battery saving it is about screen saving. I doubt that little bit of graphics for the status bar uses much power no matter what. It will burn your screen though. I think blue has a half-life of 17,000-30,000 hours (red/green over 100,000) so blue will be half of it's original brightness then. Of course you will start to see the burn much before because half brightness is pretty severe. You will start to see permanent image retention (mostly noticeable by a yellowish "stain" in the area of the static image) probably at a couple thousand hours (maybe sooner) depending on how bright you have the screen, and how sensitive you are to noticing these things.
I wish software developers, including Samsung themselves, would keep this in mind when creating mandatory static images on their devices such as status bars etc. OLED is great but it is not LCD when it comes to longevity, and care should be taken accordingly.