With Xodo latest color picker, you can add your favorite color as follows.

Method 1:

1. Go to custom color picker by swiping to the first screen:

2. Choose the color you like by tapping on the button. After finished, tap DONE to add those colors to favorite list.

Method 2:
1. Long pressing the color you like in Recent color section and Standard color section. Then pressing Star button to add them to favorite: