Highlights - where the light first hits your image. We show a highlighted area by absence of color or a lighter color. Highlights are normally shown by using Copic colors ending with numbers like OOOO, OOO, up to markers ending in 03.
Shading - Where the light hits less than than another place on the image, but not the darkest. AKA mid-tones. This is your main color normally a Copic color number about 2 numbers brighter than your highlight color or numbers ending in no higher than a 5-6.
Shadows - Where the light hits least or not at all. This is normally shown by using a grayer or darker color.
Cast Shadow -A cast shadow is formed due to light not being able to pass through an object. The cast shadow simulates the image or object that is blocking the light. A cast shadow is not always gray, but a muted or darker color of the image it is revealing itself on. IE: The tree is casting a shadow on grass that was colored with YG03, I colored the shadow with a YG63 which is a grayer shade of YG03. I could have reached a similar outcome by shading with a N3.
Let's start by doing a light study prior to starting to color our image. This is done with a Lighting Guide Template which can be made on a transparency with lines drawn on it. The lines represent the light. As light travels in straight lines.