The recommended service interval for this vehicle is every 10,000km. Roughly speaking, every second service is a bit more involved, so that suggests it’s more of a `major’ service. Either way, skipped services on a vehicle like this will be false economy in the longer term.
On the timing belt front, there’s good news and bad. The good news is that both the turbo-diesel and petrol V6 versions of the Colorado both use timing chains, so there’s no need to change a timing belt. The bad news is that the petrol version uses the Alloytech V6 which is renowned for its ability to stretch its own timing chain to the point where it needs costly replacement. This is a known fault with this engine.