Most rental car companies work on a 24-hour clock rental period. If you return the vehicle later than your pick up time you may be charged an hourly fee for each hour over the daily amount. Check with your rental car company to determine your hourly rate. Most vehicles fit into a vehicle class and when renting the company only guarantees that class of vehicle. It depends upon the number of vehicle options within a class.
Color is determined by availability.
If you have specific needs, you should check with your renting location to determine if you can make a special arrangement.