What means the color yellow on iPhone battery percentage?

Your battery is in low-power mode. Charge it and it will turn green eventually. You can also disable by entering the control panel and selecting the battery icon.

The yellow color is warning you that you need to charge the battery. You do not want it to be completely depleted as that is bad for lithium ion batteries. I rarely let mine go beneath 40%. Yours is clear down to 15%.

You can also manually turn on/off low power mode.

Learn more about Low Power Mode here -

https://support.apple.com/en-us/HT205234