Here is the second still life setup from my Foundations I Class. The emphasis has been on value and shape. Developing the ability to see. Sounds simple. Typically there is not too much trouble with the bottle and its shadow. More trouble with just local tone - no shadows - how to make that interesting? Well, just add an orange to the setup and have them paint it in a complementary pair such as blue and orange.
Light and Shadow
Yes, you can see the core shadow and the reflections and they can be confusing. Everyone says they can see what is in light and what is not. But most everyone painted the shadow side of the orange much too light, but not the bottle so much. The light sides were often too dark. Seems that as soon as colour is introduced, value judgement disintegrates.
Grey Scale
And the Notan might look something like.....
Thumbnail on Newsprint
It is not easy to paint light and shadow (a predominant mode) when one cannot distinguish what is in light and what is in shadow. Seems like the colour in all its glory confuses all those rods. The Practical Colour Wheel shows approximately the local colour and value relationships for paint pigments.
On Value 5 Grey
So Ultramarine Blue is dark and Orange is light. Somehow we get it for the blues, not for the warm colours. Squinting and value matching sure will help this. Practice, practice.