To view Battery Percentage on any phone with the Face ID "notch" at the top of the screen open the Control Center by dragging down from the battery icon, or add a battery widget to your Home screen. If you examine the status bar at the top of the screen you will observe that there is no room to put the battery percentage.
See→Use widgets on your iPhone and iPod Touch. The widget is quite useful, as it can display the status of your Apple Watch and your Bluetooth earphones in addition to the phone.
It currently uses green for greater than 20% or low power mode off, yellow for Low Power Mode, and red for under 5%. If you added more colors I think you would lose the low power mode display, which is quite useful.
You can suggest changes to Apple→Product Feedback - Apple, and you wouldn’t be the first to raise this question.