Can form curves with masonite or other types of bendable boards. Depth should be at least 5 inches, it's personal preference beyond that. I would form 1-3" above the patio and several inches (3") below ground for stability.
Mixing dye into the concrete to match the color of the already poured patio may be harder than you think. However, I know nothing about the stamping process. Make sure to use the proper edging tools when you pour, and make sure to tap the forms in order to remove air pockets from the exposed sides of the finished project.
Matching the color with any existing cement is hard to do. You may want to pour the curb using a good complementary color to the color of the patio. I don't know what color your patio is, but sometines a good complementary color looks better than a shade-off almost match. Just a thought.