I have a 1995 GMC sierra truck smoke when it stop at red light.

White smoke is coolant leaking into the cylinders, White with a blueish tint is oil burning. Black smoke is too much fuel.

Question, does this pickup blow black smoke on idle or at a stop sign. If so check your map sensor might be plugged up or unplugged or bad.

If the check engine light is on, you would need to check the computer for trouble codes first.
If the engine is fuel injected, it could be a bad injector. If it has a carb, it could be a sticking choke or an internal leak.

Pull the vacuum line of the transmission modulator valve. If there's oil in the line, the diaphragm in the modulator valve is cracked and engine vacuum is pulling trans fluid into the engine intakee

Valve stem seals. Oil is thicker when cold (hence no smoke when cold) Vlave stem symptoms are when engine at idle (high vacume) oil is pulled through the guide seals, when you take off from lights get bit of smoke until oil is burnt. Ring symtoms is it would be blowing smoke all the time especially at higher rpm for extended time ie: driveing. Have you thought of trying a thicker oil? what oil grade do you currently run?

A bad coolant temp sensor for the ECM will cause that problem ... they are cheap and easy to replace. One other possibility is a piece of carbon stuck in the EGR causing an internal vacuum leak ..
Good luck.

Try adding some engine restorer to the oil and see what that does.Thanks for asking and I hope it helps.

It's time for a Italian tune up!

The blue smoke can be one of two thing.

oil drop from the valve stem (from engine head to cyclinders)
or
diesel soot collect inside the exhaust system.

===
I will take a 5-10 minutes high rpm (not redline) ran on the highway.
Pay close attention to the rear view mirror and look for black diesel soot.
=== I will use Diesel Cleen or similar product to condition the fuel. (keep moisture down)
Use the recommended amount of SEAFOAM oil additive to condition the valve stem seal/rubber parts.

I can not rule out excessive blown by.
Can you get a diesel compression tester?
When was the last valve adjustment?

Please post the actual odometer on the engine and I will follow up on it.

If the smoke is white smoke and you have it after it has run for a while or when you stop at a stop sign or light it is one of two things or possibly both. I have a 2003 GMC 3500 that had the same proble. It turned out to be a combination of Injector being bad and Glowplugs. Injectors are covered up to 200,000 miles but not the glowplugs. I bought 4 glowplugs of the internet for 10.49 a piece AC Delco New. The dealer replaced all 8 injectors and 8 glowplugs. I paind for 4 GP's and the installation 213.00. Truck has more power and NO white smoke you might need to take it to the dealer. The test for the injectors is about 85.00 but well worth the expense

white smoke indicates a blown head gasket check for milky looking oil on the dipstick