Colobus hands have four long fingers and, at most, a stub of a first digit. Moving through a canopy at speed, a hand that closes over a branch like a hook is faster and less prone to catching than one carrying a thumb. The trade-off is fine manipulation: colobus monkeys handle food with the whole hand and do not perform the precision grips that capuchins and macaques use.
The rest of the animal follows the leaf diet. Guerezas have a chambered, sacculated stomach where bacteria ferment cellulose, in the way a ruminant's does, letting them digest mature leaves and even some toxic ones. That gut is heavy, so they rest for long periods, and it constrains how fast they can shift diet when a food tree fails.
The thumb reduction is not unique to colobus among primates, and it is often wrongly described as complete loss. In many individuals a small tubercle with a nail remains, so the correct description is reduction rather than absence.