Try to be very matter-of-fact with your son and don't get involved in arguments with him. In the mornings let him know how much time he has before he needs to leave ("You have 15 minutes ... You have 10 minutes ...") and then walk away. Many children see arguing with parents as fun, and when you take away that argument the game loses interest.
If the writing or coloring is important (if it's part of your son's homework, for example) focus on the positive with him. Let him know that writing with you for 15 minutes can earn a reward such as an extra bedtime story or a walk around the block just with you.