Not Solved. I just came back from a drive on a road where the display shows all bars, and top top bar stayed the same color as the rest, no matter what settings I changed. I tried every combination of parking and headlights on/off, and dimmer switch positions (off, low, high, full high past click). No difference.
Try it again at night, and see what you find. The color difference is weak, and I believe there are saturation and human perception effects involved. It renders very poorly on my camera and laptop display, but it was very real last night while driving in the dark.
If the difference is that weak, can we safely assume this is not a design feature, but rather a display artifact of some kind? Seems like if it was designed to impart information, it would have been made more obvious. Of course, you cannot see the red lines hardly at all on the other display, so then again, maybe not safe to assume yet.
My wife and I just got back from a trip in the mountains, and was able to get 8 bars 4 different times, here is a pic of the event. All in all, it looks the same to me.
Are you sure? In your picture I'm seeing a hint of color change on the top bar, less than I saw in real life but more than shows on my camera / display combination. Note that the OP's claimed color difference is "slightly".
I can definitely see a hint of color change in the top bar in your photo. It even looks salmonish to me. Stream's photo, on the other hand, has all 8 bars the same color (although the photo was in torch mode).
In the day display mode, when the HSI on my V gets into the PWR area, it is the same greenish color as the rest of the ECO bar. But at night, it is red/salmon. Might the same be true of the top battery bar?
I confirmed this yesterday coming down from the mountains. The top bar is the same salmon color as the "PWR" label, when the headlights are on. Turn the headlights off, the the top bar is the same color as the others. Oncoming traffic thought something must be wrong, flashing my lights like that! And a bonus: if the battery indicator is at the second-from-top bar (top bar dark), the second-from-top bar is about halfway between the salmon of the top bar and the greenish of the rest of the bars. Again, only when headlights are on.