function to control an rgb led

it's working but is it possible to fade from 0 (black) to randoms colours automatically ?
Exemple:

• Push button --> start led sequence from 0, 0, 0 (R, G, B)
  
• calculate new random values for each led
  
• fade from 0, 0, 0 to new values a, b, c
  
• When fade done, calculate new random values
  
• fade from a, b, c to new values d, e, f
  
  

I have a big logical problem for that. I presume that it is not so difficult but I can't really understand how it works !