Changing colors is a right reserved for system administrators, apart from color management of spaces which is also accessible to administrators of these spaces.
You can define the colors at different levels on your platform:
At the groups level
You can personalize the color of the header background (or possibly a background image)
Click on the group settings for personalization.
At the spaces level
You can personalize the background color of the header (or possibly a background image)
Click on 'Look and colors' in the options of the space to change the colors.