Follow steps below to change edge color bar of iphone X :
Step 1 : Login into your Appy Pie account
Step 2 : Go to My Apps
Step 3 : Click on Edit and Manage of your selected app
Step 4 : Go to Design Customization
Step 5 : Scroll down and click on Background option
Note - If you select "None" as background option, your app users will see white edge color bar on iPhone X