What processes produce the different colors we see in opaque objects?

Answer;
Adsorption and reflection of different wavelengths of light.

Explanation; 
Opaque objects are objects which we can't see through. 
The color of an opaque object is the same as the light that is reflected off it.
The color of an object depends on the waves of visible light that it reflects. When the light hits an object some is absorbed (adsorption) and some reflects, for example, white is white because all the colors are reflected back, plants are green because the chlorophyll reflects the color green, making them appear green to us.

More information

Rain is pulled by gravity which is a non contact force