Never sign an incomplete contract. Always read and be sure you understand it before signing. Also be sure it is the car you want, can afford and that it is in a condition you will accept.
With new vehicles, specify the car’s colour and delivery date on the contract. If a dealer does not deliver as agreed, depending on the terms of sale the contract may be terminated and deposit money refunded, particularly if the deficiency cannot be appropriately remedied by the dealer.
If the dealer's standard contract does not make provision for your specific requirements then you may need to insist on certain wording being inserted into the contract to protect your interests.