I am having trouble starting my 2013 Holden Colorado

Most glow-plug systems in diesel cars work on a timer system. The manufacturer will have worked out that X seconds of current will heat the plugs to a temperature sufficient for easy starting, so that X-seconds period is where the timer will be set.

What happens, though, is that glow plugs can lose their efficiency over time (and with 233,000km on board, your car is a candidate for that to have happened). As that efficiency is lost, they don't produce the same amount of heat as they used to and, therefore, the timer isn't giving them long enough to heat the combustion chamber sufficiently. When that happens, the engine becomes hard to start. The other possibility is that the timer itself is faulty and isn't giving the plugs power for long enough. The fact that your engine starts easily after the first cold start of the day, suggests the glow-plugs are, indeed, at fault here.

The other possibility is that the car's electrical system is not sending enough voltage to the glow plugs to make them work properly. This can be verified quickly and easily with a voltmeter. This method can also verify the length of time the timer is supplying power to the plugs.

In vehicles where the timer has already shut off the power to the glow-plugs before enough heat has been generated, the solution is often a new set of glow-plugs. The good news is that these aren't expensive, nor are they difficult to change if you know what you're doing. The caveat is if you aren't sure of what you're doing, at which point the fuel system on a common-rial diesel injection system can be very dangerous to tamper with due to the super-high pressures involved.