I'm updating this previous post as the Roma Pass site has just included instructional ticketing text about this change. As of March 1, Roma Pass holders will no longer be able to access the Colosseum without a prior reservation. From the Roma Pass website (sorry, the text is pretty long):
http://www.romapass.it/en/FAQ/
Under "reservations":
For the Coliseum, the Borghese Gallery and Palazzo Valentini (Domus
Romane), the ticket reservation is required, since the museum allows a
limited number of visitors. Reservations are not required for any
other museums, though it is highly recommended that school or tourist
groups make reservations, in order to facilitate entry to museums.
Please note: For the Coliseum, Roma Pass holders can make a
reservation in three ways:On-line, on the website www.coopculture.it in the "booking with Roma
Pass" section – required booking fee 2€ (with credit card)Call center at 0639967575 – required booking fee 2€ (with credit card)
At the Roman Forum - Palatine ticket offices, only on the day of the
visit (upon availability)- the ticket office will issue a free receipt
with the admission time.Free booking can be made only on the day of the visit at the Roman
Forum - Palatine ticket offices (upon availability); the ticket office
will issue a free receipt with the admission time, for that day.Roma Pass holders can make a reservation online or by calling on the
day of the visit. The booking fee is required. The online booking or
by call center will issue a note with PNR which will be changed for a
receipt with an admission time at a reserved counter at the Roman
Forum ticket offices. This receipt is not valid for entry, but only
for the time slot. Roma Pass holders, through the security checks must
go to the access gate of the Coliseum, presenting their Roma Pass,
otherwise it’s not possible to enter even if they have an time
booking. Afterwards, Roma Pass holders will enter the Coliseum through
a dedicated gate controlled by Coopculture’s staff.Roma Pass is valid only for ordinary access to the monument (Arena,
Sotterranei and Belvedere not included) Visitors with free admission
who enter with Roma Pass holders already booked an admission time, can
collect the free ticket at the ticket office skipping the queue. The
booking fee is € 2.00. For groups, all participants must hold Roma
Pass and must follow the procedures for booking groups on the website
www.coopculture.it.
There's similar text on coopculture Colosseum ticketing pages.