I have noticed that on looong, flat stretches of 40 MPH (it's different each day, though, never exactly the same speed twice in a row) driving with no wind, traffic, or red-lights, I too, can make my battery display go to green (all but one bar lit).
I think the thing to recognize is that the computer is programmed to change its "bias" or "favor", depending on the SOC (state of charge).
The farther above "half" it gets, the more it will favor electric assist or full-out electric power.
The farther below "half" it gets, the more it will favor ICE power + charging.
In my experience, it will often "go green" if I've been driving flat for a period of time, while maintaining a speed that allows charging to take place. Likewise, it will "go green" if I have been driving while it's charging, then suddenly, I go down a hill or off a long expressway ramp, suddenly causing more regen than the computer may have expected. Regardless of how it gets there, once it "goes green," it really changes its mind, and wants to discharge that battery. In fact, it's quite stubborn about it, because it's hard to make it "stay green" for very long.
Also in my experience, I find that it will often "go red" if I've been running the AC while parked, or if I have just started it cold and accellerate to the point of using a lot of electric assist (particularly since I like to drive "stealth" just before parking it for a long period, usually discharging the battery to 50% or less). But just as in my prior paragraph, I have noted that once it "goes red", it tends to really become resistant to discharging the battery any further. Again, the word "stubborn" comes to mind, because it seemingly only agrees to shut the ICE off at a dead stop or once the battery has gained a little charge to at least get it out of the "red zone."
You can "fool" it from time to time, but yes, I agree with the comments here that it generally likes to favor the dead-center on the battery. Personally, I think this is an indicator of a good bit of programming and design.