The first thing you need to ask yourself is "Do we have money in the budget to rent a getaway car or not?"
If you DON'T, then traditionally it is the best man and groomsmen's responsibility to decorate the getaway car. Often the words "Just Married" on written on the back window along with flowers, cans or more elaborate signs. If you want to depart from the reception in a decorated car, gather all the necessary supplies (and maybe an inspiration photo) then give everything to the groomsmen at the rehearsal dinner. Let them know the logistics like where and when they will have access to the car and give them a spare car key.
Find some inspiration here: https://www.pinterest.com/brandyrosejuly/awesome-getaway-cars/
If you DO, here are some questions to ask the rental company. What percentage of their business is weddings? Do they own the cars or are they booking them from another company? What will the chauffeur wear? Do they charge extra to go take pictures somewhere between the church, venue or hotel? What is the cost to extend or change the times that evening? Are there extra charges for changing clothes, unscheduled stops or cruising around until your time is up? Do they include a free photo shoot in their packages? Can you work out a deal for transporting the guest to the reception, bachelorette party and the rehearsal dinner? (TIP: Have your wedding on Friday or Sunday to save $!)
Some things to think about: If your dress is white and the getaway car is white, your dress may not stand out with the car in the background. If the groom's tux is black and the getaway car is black, he may blend in with the car. Silver is an excellent color choice for the getaway car. You will both look great!
TIP Ask a family friend or relative to "Sponsor" the limo instead of buying a wedding present!