from the explainer link, 'RGB Color – HTML and CSS Guide ’ :
The minimum value hex colors can use is `00` which is fully off, and the maxiumum value is `ff` which is fully on. The color white in hex is `#000000` or `000` for its shorthand notation and the color black is `#ffffff` or `#fff` .
from the exercise:
Replace the word black in our body element's background-color with its hex code representation, #000000.
(…the latter seems to be correct)