VALUE Toast gradient. Human eyes can perceive 255 distinct levels of values (shading). If someone can find toast with 15 different levels, surely you can have more than just 3 or 4 in your drawings and paintings.
This picture is a great example of SPACE because of the obvious distance around the banana. The positive space is the banana and the negative space is the backdrop. The backdrop behind and around the banana creates the blank feeling of space.