Hey there!
I would say the answer to this question is Movement.
If you look at the principles of design the definition of Movement is Movement is the path the viewer's eye takes through the work of art, often to focal areas.
In Cathy's design, it states that when viewers view her website their eyes first fall on the brand name, and then they view the bright images on the page.
Based on the information given you can conclude that the design principle that Cathy used was Movement.
I hope this helps,
Have a great day!