1. Stack 3 or 4 collard leaves flat on work surface. Roll tightly into a cylinder, hold together, and thinly slice to make narrow strips. Coarsely chop strips once sliced. Repeat with remaining collard leaves, and transfer to large bowl. Stir in carrots, onion, and bell pepper.
2. Whisk together vinegar, sugar, oil, mustard, celery seed, salt, and pepper in small saucepan, and bring to a boil, whisking to dissolve sugar. Remove from heat, and pour hot vinegar mixture over collard and vegetable mixture. Stir to coat vegetables with dressing. Season with salt and pepper. Cover, and chill up to 4 hours or overnight for a great taste.