Probability question, What is considered a unique feature?

Suppose I wanted to find the most unique features in an image and then find that same feature in another image.

Image 1 has unique colors and no unique line lenghts.

However Image 2 has unique line lenghts, but no unique colors.

What is considered a unique feature in this case? The lenghts? The colors? Or an in between of the two?

I can't figure this out.

Im trying to match features between images.