In reality, shadows are very rarely black or pure grey. As well as changing in line with the strength, colour and quality of the light, the characteristics of shadows change with their surroundings. The more complex the surroundings, the more complex the shadow.
Shadows are not just an absence of light. Instead of thinking of them in terms of what they are not, think of them as what they are: the addition of the qualities of the casting object(s), the qualities of the light and reflected light and the qualities of the surface that they are being cast onto. These qualities include opacity, (solidity), tone, texture and hue (colour).
Some general rules of thumb are this:
1.The stronger the light, the darker the shadow.
2. The more solid the object, the more solid the shadow is (particularly its edges).
3. The more angled the light, the longer the shadow.
4. The longer the shadow, the more it interacts with its surrounds (shows variation).
5. The more textured the cast surface, the more jagged the shadow.
6. The more reflected light, the lighter and/or more uneven the shadow.
7. The more absorbent the cast surface, the darker the shadow will be.
8. The further from the casting object, the larger and often lighter or less crisp the shadow will be (unless the light source is very strong and/ or the objects are very solid and small. You may not see any variation in a still life scene).
In terms of shadow hue, (colour), consider that in that dark tone, there is also a blend of the hues of the surface and the hues of the casting object as reflected light.