Essentially, impurities, the material being burned, and uneven combustion. A clean hydrogen flame is almost invisible. When burning a propane flame it is blue and purple when it has good even combustion and ionizes the gasses in the flame and it turns blue. Orange flames are more uneven combustion as well as different fuel being burned.

By changing things like how much gas and how much is let out as well as oxygen content you can change how a flame behaves. For a fireplace you want pretty long yellow fire, so you adjust the pressure and mixture for that. For a stove you want hot and for the fire not to go a foot above the stove, so you adjust for that.

So for gas fireplaces is it not burning natural gas provided by the gas company via a line from the street?

It is. Hotter more complete gas fires are blue. "colder" less complete gas fires are orange.

What do you mean by complete ?

There is a perfect ratio of fuel to air that is perfect. For example, 2 parts fuel to one part air. If you stray from that perfect mixture, you'll have different flame properties

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In a wood fire (and also similarly in a candle flame, or the flame of a simple lighter, or a bunsen burner with the air valve closed, number 1), because the air has to flow into the flame from the outside, there is not enough oxygen in the flame to burn all of the fuel.

Because the fuel in all of these cases is a carbon-based material, one of the products of this incomplete combustion is just... leftover solid carbon, which forms into tiny particles of soot. And because these tiny particles are in a hot flame, they are also hot, and so glow a bright yellow.

On the other hand, in a gas burner for heating, or a bunsen burner with the air valve open, the gas is mixed with air before it enters the flame. That way, there is enough oxygen available to quickly react all of the carbon and hydrogen in the fuel to carbon dioxide and water (this is what is meant by "complete combustion), so there is no soot that can glow, and you get both a hotter flame, and also more heat per each unit of fuel.

i've always known blue flames to be hotter than orange flames and that orange flames actually indicate less oxygen. can't tell if my life is a lie or not

Because gas fireplaces are not intended to be an efficient source of heat, they are made to replicate the appearance of a wood fire.

A blue flame means the gas is being burned efficiently with the correct fuel-to-oxygen ratio. A yellow flame means there is too much fuel and not enough oxygen, so the fuel does not burn completely. Some particles of carbon remain, which incandesce, glow yellow, and are released as soot and smoke.

> For a fireplace you want pretty long yellow fire,

I get that many or most people want just the appearance of a fire. I'm on the side of wanting heat. But I also love the smell of a real wood fire and the crackling and popping, which is most likely way less efficient than you could get with the proper mixture from a gas one.

A fireplace is heavily a décor element, and thus intended to look pretty, even at the expense of efficiency. Some gas and electric ones will even have speakers built-in to provide the sound of burning wood. Typically, you can adjust the mixture in a gas fireplace to be more efficient, though doing so will sacrifice much of the décor element.

You can also get gas space heaters, which are intended to produce heat without looking pretty, and will generally be adjusted for a high efficiency burn.

Orange flames in a gas burning device can indicate improper fuel-oxygen mixture as it's not combusting properly. This can lead to carbon monoxide poisoning. Call your utility/gas provider/maintenance personnel and get it checked for the problem!

Wood-burning fireplaces have orange flames because of the sodium present in wood, btw!

A blue flame means the fuel (gas) is combusting fully - complete. Incomplete means not all the gas is being consumed in the reaction with the excess being released into the atmosphere.