Hello @rontha ,
I also encountered with the same issue in my xanadu instance there is no theme builder so customization of having different colors themes of the Header Banner color to differentiate between instances gets little difficult..

How I solved :
1) I make a custom color theme in "sys_ux_style" table and there I pasted the color pallet which i design from another website.. 

2) attached that custom color theme in my custom theme 

You can generate the json color pallet from any third party webside..
If you want I will share the steps too..

If solution is helpful mark as helpful and accept the solution..