There are a few ways to change an object's color.
1. If the object uses the Default Material (which is applied when drawing a new object), the object will get its color from the color property found under the object's properties.
The reason for this is the Default Material is configured to use the object's color as part of the material's diffuse color. If we look under the diffuse property group of the Default Material we can see where this is set:
When 'Diffuse Color Source' is set to 'Material + object color', it means add together both the object's color and the material's color (which in the above case is black, so it contributes nothing).
For other materials you might create, the material may be configured to only use its own color ('Diffuse Color Source = Material') at which point the object's color is ignored and only the Material's color is used.