Colorado has an additional entrance fee per vehicle. Booking is online through Reserve America and most campsites are reservation only. You can book same day and up to 6 months in advance.
The daily entrance fee is $10 in 2023 though Colorado does offer an annual entrance pass for $80. You can buy the annual pass online, at state parks, or at some retailers, such as REI. Even though it is called an annual pass it runs for 12 months from the date of purchase.