As with any four-wheel-drive that is approaching middle-age, you need to treat every Colorado on its individual merits. How has it been driven? Has it been used as a work truck? Has it spent every weekend off-road?
Beyond that, the Colorado is subject to the same concerns that challenge all owners of modern, common-rail diesel engines. Those start with the soot filter which can clog up and require manual cleaning. The solution is to make sure you do at least one decent freeway journey every three or four weeks as a means of allowing the filter to regenerate as it’s supposed to.
The other thing that catches many owners out is a build up of black gunge in the engine’s inlet tract. The black ooze is a combination of oil mist from the crankcase ventilation system and soot from the exhaust (a modern engine consumes a proportion of its own exhaust gasses as an emissions-reduction measure). When these two compounds mix, the black, sticky murk is the result. If it’s bad enough it can trigger a check-engine light on the dash and can lead to poor running and fuel economy. Left long enough, the whole intake system will need to be removed and manually cleaned. An aftermarket oil separator stops this is it traps the oil mist before it can enter the engine and is a good addition to these vehicles.