You can use the iPhone's clock to trick this game into thinking you've been waiting days for your Smurfs' crops to grow!
To do this, go into your iPhone settings and set the time and date forward to when your crops are due to grow - you may need to turn off the automatic time. Just to be clear, that is your iPhone settings, not the in-game ones.
Painter Smurf is able to paint objects blue, red, and white when he has been unlocked. To unlock the other colours you must complete the corresponding image in the Painter Smurf's mini-game at a 'Close To Perfection' rank (colors cannot be unlocked in Practice mode).