Question

Which design element indicates the relative lightness or darkness in a design

Answer 1

i'd say it'd have to be value 

Answer 2

Contrast

Step-by-step explanation:

Contrast is the level of difference between two design elements.

Contrast also has a great effect on readability. This is why you see novels and other publications printed as black text on a white background. Imagine if they printed using light gray text on a white background? The contrast would be lower, and reading would become difficult. So if you are using typology, be sure to intensify the contrast.