You could make an "Ice magician" someone that can lower temperatures and a "Fire Magician" someone that can raise temperatures.
(If you want realism we need to remember about the law of conservation of energy. Therefore, lowering a temperature somewhere will raise it elsewhere, and vice versa.)
The grade and speed of which would probably be determined by the raw power, ability, skill or level of training your Magician has reached or possess.
Just because we don't have a defined absolute limit on heat (minus taking all energy in existence and concentrating it into one point) doesn't make the lack of heat an inferior ability. On the contrary, you get to freeze molecules on an atomic level. Even cosmic beings of unimaginable power must consist of something, even if it's just a thought, thy are made of energy. Energy can be brought to a complete standstill at certain temperatures.
Adding energy, or "making hotter" excites molecules. Enough energy causes them to break apart, combusting in many cases. Lets face it, you wont feel a difference if you touch 1000 degrees Celsius or 10,000 degrees Celsius as long as its on the same point. You can, however, increase the range the heat radiates to.
Its possible to take this further, and define your "magic" as transfer of energy. In other words, a "Fire Magician" in essence draws energy from the area around an object into that object. The result is a hot object, but an area, or other objects in that area, devoid of their energy, leaving them cold, with the opposite being true for the other side of the coin.
Remember, you cannot have ice without water (but you can also solidify other liquid substances at low enough temperatures). You can use moisture from the air, but that will leave the air dry. This can be incorporated as a side effect and ability.
With fire its more complicated. You need fuel and a source of ignition. Many gasses and gas mixtures will combust without a source of ignition if heated enough. You will need an oxidizer (oxygen works well) as well as a second substance that likes to oxidize (like metal) to create a ball of flame. This could be anything from VERY fine sand to biological substances, coal, certain gasses and the like. For one of these Magicians, the ability to compress and depress air, create magnetic fields, "molecularize" items by snipping them up into small components, as well as air flow manipulation can assist in creating the form of fire they wish. Without all these components your "Fire Magician" could just as well walk around with an aerosol can, Molotov or paraffin-dipped charcoal and a lighter.