I was reading that link, are you completing the form DR0167?

Documentation required to claim the credit
Any taxpayer who claims the credit must include the following attachments with their Colorado income tax return.
This documentation is required regardless of whether the taxpayer is claiming the credit as a purchaser, lessee,
vehicle owner, or financing entity. The following documentation must be submitted for each vehicle or conversion for
which the taxpayer is claiming the credit:
 A completed Form DR 0617;
 A copy of the invoice, purchase agreement, or lease agreement for the qualifying motor vehicle or truck;
 For financing entities that accept assignment of the credit, a completed Form DR 0618.
A taxpayer claiming credits for multiple qualifying motor vehicles, truck, or conversions must attach to their return
the above listed documents for each qualifying motor vehicle, truck, or conversion.