They are all round and black. But the selection of the proper tires for the SL-C is of paramount importance, as they are the interface between the car and the road. All acceleration, braking and roadholding force is transmitted through the tires.
When selecting wheels and tires, the builder should carefully consider the effect of wheel/tire weight. A wheel/tire combination in the same size wheel and tire can vary by as much as 25-30 lbs across manufacturers. That can mean a 100 lb or more difference in the overall weight of the car, but worse, adds weight that is unsprung, and also weight that has an effect on rotating mass, which affects acceleration times (and braking). In general, tires (and wheels) are heavier as their diameter goes up, which is one good reason to avoid the very large wheel diameters available. The SL-C isn't a donk.
Before you commit to that set of blingy wheels, carefully research the weight of the wheels, and the tires you are considering.
Unsprung weight is a measurement of the weight of everything outboard of the wishbones or suspension links, plus 1/2 of the weight of the wishbones or links and spring/shock. It has a great effect on handling.
The more weight outboard of the car, the more force bumps exert on the suspension (and ultimately the chassis). This force must be dealt with using springs, shocks and anti-roll bars, and the more force, the more difficult it is to keep the tire planted on the road. This is especially true of lighter weight cars. In the example above, if the car weighs 1000 lbs, a 2G bump would result in a vertical force of 10% of the car's weight. This will at the very least reduce the grip of the car, because the weight of the car is what keeps the tire planted, and pushing a car up into the air with that much force will inevitably reduce the weight on the tire, and hence grip.
The SL-C is designed for the stock Corvette C6 Z06 tire.
A complete discussion of wheel sizes, and the recommended tire sizes can be found at the Superlite webpage here.
Using the stock Corvette C6 Z06 wheels or wheel sizes, ensures long-term tire availability, as there are many of those cars on the road.
For track use we recommend the following wheel sizes:
Front: 18 x 11 (may cause some minor clearance issues with steering at full lock, full suspension droop, but this normally isn't an issue for track work)
Rear: 18 x 13
For track use we recommend the following tire sizes:
...or similar sized slick tires. Hoosier makes a DOT-legal tire that is popular at the track, as it has relatively long wear (for a track tire) and superior performance. These are among the lightest tires available as well. Some people have had grooves cut in them to allow for at least a modicum of damp driving. The standard Hoosier R6 is a dry tire only, and should never be used in wet conditions as lacks any way to evacuate water under the tire.
Note that for race cars, the recommendation is for 18" wheels, as these have a greater range of available race tires.
For the street, the overwhelming majority of SL-Cs use the popular Nitto Invo tires, which are quite inexpensive for the size and stickiness.
One builder that has 19/20 wheels is using Continental tires, and another popular tire is the Michelin PS2 series, though they are among the most expensive tires.
Tire Rack has online tools to search for appropriate tires. If you are using the factory-provided wheels, just tell the site you have a 2011 Corvette Z06 and search that way.
Note that the wheel and tire sizes, as mentioned below in a comment, are now available in a wider range from the factory. Be sure you read the Wheel/Tire page on the Superlite webpage (link above) before you buy tires, so they are compatible with the exact wheels you ordered. Another way to think about this is to select the wheels that have the tire sizes you want to use. Not every wheel combination has an ideal range of tires.