Learn While You Play
The most useful design principles are the ones you teach yourself.
How do you do that? You play. You notice what's happening. You ask Why?
What if I create a second block by reversing the colors in the first?
The green in the center looks darker. But I know it's the same!
I think my eye, or rather my brain, is mixing some of the black into the green and making it darker.
My new priniple: By placing black fabrics side by side with light ones I can make the light ones appear darker.
Now they look closer to the original green.
Corollary to my new principle: If fabric is mixed in with a lot of black, I have to chose a lighter color to keep it from appearing too dark.
The purple designs look tilted to me.
The arms of the design don't line up, they are staggered. However, the arms join up and so they create a tilted line.
I'm not sure how to state this as a principle. Perhaps you have a suggestion. In any case, I know I can use it when I design.
If you start playing, I'm sure you'll find all kinds of ideas and principles you can use when you design.