Buick parts diagram

Question edited for clarity and toss in some CAPS.
Question moved to model category.

Shown in the parts schematics

https://www.google.com/search?q=1999+Buick+Regal+fuel+lines+diagram

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After you find it , before replacing ,disconnect the hose that goes to the back of the vehicle ,to vapor canistor . With the engine running place your finger over the end to see if it has suction on it. Too many times autozone recommends a part be replaced and their is nothing wrong with it . If it's not leaking suggest you take your vehicle to a qualified repair shop an have the evap system smoke tested .
The purge solenoid is on the left side of the engine , below the intake .

Only two are fuel lines, the main feed to the engine and the return line from the engine to the tank. One other gas tank line will be the EVAP line for sending gas tank fumes from the charcoal canister to the engine's intake manifold.
I think the other lines would be your brake lines to the rear wheels.

YouTube has many videos on repairing rusted fuel lines. Most people use the lines made of plastic in their repairs-takes special fittings though. These plastic lines have become very common now. In fact my '92 LeSabre has mostly plastic lines.

The Chilton/Haynes repair manual we got for our 2004 Trailblazer has a fuel diagram. So I believe the same type manual for your Colorado should be no different as to information. Did find in research that there are plastic parts on the fuel lines that run under the air intake. Over time they become brittle due to engine heat since they are on top of engine but under the plastic air tube and are prone to leak over time. Not hard to fix, just a pain to remove the interfering parts.

Where is it leaking? If its at the fuel rail fuel pressure regulator you may need a new regulator if leaking on the metal part. If its leaking at the rail side its just the "O" ring. Relieve fuel pressure(push in valve under plastic cap right side beside alternator on rail. Remove the torx screw holding regulator bracket. Pry out and replace "O" ring, reassemble. If its not the regulator it may be a fuel injector, you need to remove the fuel rail and replace both the upper and lower injector "O" rings. This however needs the intake manifold removed to repair.

You could have either an injector that is stuck open or a fuel pressure regulator that has developed a leak. A leaky injector will usually only leak fuel into one cylinder as it is located just above the intake valve in the lower part of the intake while a leaky fuel pressure regulator will leak fuel into the front end of the intake manifold [behind the throttle plates] and the fuel will acuumulate throughout the upper intake plenum. When a fuel pressure regulator develops a leak the fuel that is escaping from it flows through the small 1/8" diameter plastic vacuum line which goes from the top of the regulator to the intake manifold. Even a trace of fuel in this line means the regulator is bad. I've seen both leak items leak but probably replace 10 regulators leak for every one injector. Good luck, hope you find your problem!

sounds like a fuel injector is leaking they have seals
also check the fuel rail for leaks there should not be any fuel coming from inside the engine, unless u have a crack in the intake someware

leaking valve or head gasket leaking

Check out the fuel filter, also look for a fuel leak in the manaflod area. When you go to start it, turn the key on for 10 seconds before you try to start the engine and then turn it over and see if it doesn't start easier. If the fuel pump is getting weak it gives it a chance to build up pressure.

The fuel lines are steel except for the plastic ends where they need to be flexible for the push-on type connectors. If I am right, those plastic ends pass just under your cabin air intake. Just below that, is a common bend where the steel lines usually rot. If you have high miles they may be baked, or if you had recent engine work they may have been flexed to one side hard enough to split one the three. If one is leaking it is best to replace them all. Don't take chances with gas fumes or leaks. Do it right away!