Form and Color Sketch

I've been doing a lot of blocky and straight-edged stuff at work lately, and I need to break out of that. So I've started sketching out some more organic forms in my sketchbook. I'm taking some of them and rendering the forms using photos as color and material reference. I'm beginning a personal vehicle project with a couple of friends, and I need to work on form and rendering. It's been a while, and I've gotten a bit rusty.

This one is from a photo I took of a little Corvette show they had in the parking lot of an ice cream "festival" my wife dragged us to back in June. I guess the day wasn't a total loss if I got a good sketch out of it ;)