So using an Arduino I fed 5v to a white LED, nothing happened to it. I fed 5v to a yellow LED, and within a few seconds it started to smoke and burn itself out. My first question is how toxic are these fumes? Second, why do different colored LEDS have different voltage/current maximums?
Toxicity is in the dose. Just about everything is toxic at a high dosage.
I have never seen an LED manufacturer warn about breathing the fums of a smoking LED. I have smoked enough LEDs to know what it smells like. Not likely acutely toxic.
White, blue, and green LEDs are usually Gallium indium nitride with about a 3v forward voltage. red, orange yellow (amber) are usually Aluminium gallium indium phosphide with about a 2v forward voltage.
Current for AlGaInP range from a few mA to 700mA. GaInN from a few mA to 1500 mA.
When you say "I fed 5v" I would need to know HOW you fed the 5v in order to guess as to why one burned and the other did not. Both should have burned if they were connected to a 5v source capable of more than an amp.
Well I connected the 5v and ground to the LED, and for some apparent reason this one started burning... Trying different colors except yellow, none of them had the same issue. Manufacturing differences?
The power supply for your Arduino must be low wattage. Connecting an LED to a 5V power rail should make them all go up in smoke.
My guess is that the LEDs you used were not high brightness with a low maximum current.
I took a 20mA red indicator LED and connected it to a 600 mA variable power supply and slowly increased the voltage. It turned on at 1.8V and dimmed at 2.4v. As it approached 5V it dimmed even more.
On the other hand these two Luxeon Rebel Blue LEDs burned running at their max rating of 1 Amp due to no heatsink.
I have a feeling that you've connected your LEDs directly between the Arduino's output pin and ground. You're not supposed to do that - most microcontroller's output pins aren't designed to drive the current required to light a LED. You might end up burning your microcontroller too. You might have been lucky to have a microcontroller that somewhat limits the output current to a level that was safe for the white LED, but not for the yellow one.
Regarding your second question, your white LED is as close to your yellow LED as as a Chevy is to a Ford. They're just built differently, and will behave differently. That's why each electronic component has a datasheet - and why each car has a manual.
Regarding your first question, exactly what's burning depend on each manufacturer's process. In case of doubt, assume it's toxic. But you're alive, so it wasn't probably that bad :-)
Toxicity of LED Smoke
According to the journal named Environmental Science and Technology LEDs contain lead, arsenic and a dozen other potentially dangerous substances.
Especially red LEDs contains 8 times the led found in a yellow LED. When converted to smoke is a neurotoxin and exhibit significant cancer and noncancer causing properties due to the high content of arsenic and lead.
According to The Scientific American
Ogunseitan adds that while breaking open a single LED and breathing in its fumes wouldn’t likely cause cancer, our bodies hardly need more toxic substances floating around, as the combined effects could be a disease trigger. If any LEDs break at home, Ogunseitan recommends sweeping them up while wearing gloves and a mask, and disposing of the debris — and even the broom — as hazardous waste. Furthermore, crews dispatched to clean up car crashes or broken traffic lights (LEDs are used extensively for automotive and traffic lighting) should wear protective clothing and handle material as hazardous waste. LEDs are currently not considered toxic by law and can be disposed of in regular landfills.
Why do different colored LEDS have different voltage/current maximums
An aticle at https://en.wikipedia.org/wiki/Light-emitting_diode#Colors_and_materials explains that the materials that a LED is made up of effects the color and thus the voltage drop / requirements.
For a red LED the material used is Aluminium gallium arsenide (AlGaAs) or Gallium(III) phosphide (GaP) which has a voltage drop of 1.63 < ΔV < 2.03.
Similarly for a Violet LED the material it is built up of is Indium gallium nitride (InGaN) and the voltage drop is around 2.76 < ΔV < 4.0.