If the car has no defects but you simply do not like it, the car rental company is not obliged to comply with your request.
Of course, you can still ask at the time of collection whether another vehicle is available. In some cases, the rental company may be accommodating and will comply with your request. However, make sure that this is not a paid upgrade.
If you have an accident with the rental car, the rental company will provide you with a replacement car if possible. If minor damage occurs to the vehicle, the rental company can either exchange the rental car or have it repaired. To be covered for such cases, we recommend that you always book sufficient insurance cover.