1. If you want to add background color alone you can do it in the notification definition.
Notification -> What it will contain(tab) -> Message HTML -> Above the editor we have many options to make use of, one of them is background color as highlighted in the below screenshot.
This would help you to update the background color of that particular notification alone.
2. If you want to maintain same colors, layout, logos etc in multiple notifications then Email layouts would be the best option. Here we have two ways one is default which would resemble like traditional notification where you can add background colors, insert tables etc.
If you click on Advanced check box then you can write CSS styles in Email Layouts directly which could be anything of your own interest or your client's requirement.
Please type Layouts in the left application navigator then you will find Layouts under System Policy Application and Email module.
Please mark as Correct or Helpful if it is so
