If your battery is full and your iPhone shows a yellow color, you do not have to be scared, everything is still in perfect order with your phone.
We all used the colors of our phone through the years, so the red was always undesirable and it was a reserve. That is why it is also logical that the yellow is now called alert. But it’s just because the phone is in ‘low power mode’.
This is not a bug, but a phone feature that can temporarily reduce power consumption, exclude some background processes and visual effects to keep your phone running longer until you can recharge it. Many of us do not intentionally include this option, but the phone warns us when the battery drops to 20 percent to include this “low power mode”. When it is active, the battery changes the color in yellow and we often do not register that change, and mode remains on until the battery reaches 80 percent.
If you turned it on manually, the battery icon will remain yellow until the battery is fully charged.