This exercise teaches blending and achieving contrast. It is an introduction to how oil paint feels and moves around with brushes and medium.
The first sphere (indicated YES) has all the attributes you want in your sphere. Keep your darks dark, your lights light, and a smooth transition in between.
The rest of the spheres are how NOT to paint your sphere. The one with no Liquin looks okay, but without medium the paint is usually much harder to move around, you’ll use more paint. The edges have a drier look.
The dark will overtake the light if you don’t use a paper towel to clean out your brush as you work.
There are no mistakes you can’t fix. Use a paper towel like an eraser. At this stage it doesn’t matter if your circle is round, or if it’s perfect. You’re just learning to move the paint around.
Paint at least two of the YES spheres, varying your use of Liquid. Again, this exercise will give you practice with blending, getting used to the medium, and keeping your darks dark and your lights light.