2005 Colorado RPM's flucuate from normal RPM down to 500 rpm usually after I first start the truck. Then after running for 10-15 minutes this issue goes away. The truck starts fine. This problem only occurs on a cold start up and after I drive to a point I have to stop and push in on the clutch, the problem appears. It will flucuate about 3x and then goes to the normal RPM setting and will not do it again, until the truck is started again from a cold start.

in neutral your idle should be anywhere between 700-900, in gear it should be down around 500.

COULD BE TPS (RPM) SENSOR,CLEAN THROTTLE BODY.

I am not sure about your model but cars used to have a solenoid type device at where the throttle rests and usually has 1 wire running to it as the unit itself is grounded. This solenoid is usually activated when you kick the ac on. I first discovered this on a 68 Firebird that my family owned. They are also adjustable but do not adjust if you can not see it activate. You would need to trouble shoot why it is not kicking on when the a/c is turned on. You will have to look up your specific model and it may be under idle solenoid or another name depending on the manufacturer. I hope this helps

DId the Check Engine light ever come on?? Normally with such a major problem the check engine light should of activated in which case you can take it to a Autozone or most auto parts stores adn they can plug a Code reader into your system download the codes/messages and it should pinpoint or at least point you in the correct direction as to what is causing your issues.
I have had to do that a couple times with my 2005 Chrysler T&C due to a O2 Issue, and a Catalytic convertor issue.

Did you clean or replace the iac valve?? If so,start inspecting all vacuum hoses & fittings,gaskets and components that operate off of vacuum.If you cant locate a leak visually, connect a 1' long piece of rubber hose to the valve on a small,handheld propane torch.Open the valve slightly so propane comes out,start the engine and let it idle then run the hose across all of the components listed above.If the engine idle surges upwards,the component you just passed over has a leak.
A defective tps can also cause a high or flucuating idle.

Replace the idle valve with a factory one and remove the negative battery negative cable for 5 minutes to reset the idle control system to base line settings.

That sounds like a possible vacumn leak

How does the engine run otherwise? Any stumbling on acceleration? Any check engine lights on?

When the engine is racing, does it stay at a steady RPM or does it flucuate up and down the RPM range?

this is an idle valve after 1996 the ecu control the idleing from the engine with this air by pass if this valve is geting bad it can get stuck open or closed or dont working right is an electric selenoid ......... listen for air leaks ....... wen you push the clutch you push the brake too check the brake booster

Could be a sticky EGR valve, or a large vacuum hose off or burnt.