In this tutorial I will explain all about drawing and rendering car in Adobe Photoshop.        
       
First you have to find some drawing, or to draw your own. Open picture in Photoshop. Insert new layers (ctrl+shift+n), place it same like me, and name them: back, shadows, brakes, rims, tires, motor, lights, body, white, dark, shines, lighting. Use black brush and color selections like on picture        
       
Select car body in 'body' layer, and color them. When you working with document which have more then 10 layers you can to go to layer's options and decrese opacity to ~60% to see some results and then move to 100% opacity again.        
       
         
       
Your borders of carbody have some sharp edges, fix it with smodge tool, but you have to be concentraited when using smodge, (best presure is about 10%).        
       
On bottom of car place some shadows, just a little. With dodge/burn tools (layer 'body') place shadows on car.