In Colorado, the burden of proving that a will is not valid falls on the party contesting the will. This is because once a will has been accepted by the court, it is considered prima facie evidence of its due attestation, execution, and validity. Therefore, the contestants of the will have the responsibility to establish grounds for contesting the will, such as lack of testamentary intent or capacity, undue influence, fraud, duress, mistake, or revocation. C.R.S. § 15-12-407.

In Aquilini v. Chamblin, the court upheld the principle that in a contest of a will which has been duly admitted to probate, the burden of proof is on the contestant to establish his grounds of contest. The court affirmed that the probate is held to be prima facie evidence of the due attestation, execution, and validity of the will, and the burden is upon the contestants. Aquilini v. Chamblin, 94 Colo. 367, 30 P.2d 325 (1934)