The color of the flame from a gas furnace can be used to diagnose problems in the furnace itself. If it is in good working order, the flame should be blue, with only very little yellow at the tip of the flame's inner cone. If the flame is mostly yellow or orange, this signifies a major, and potentially dangerous, problem.
To burn, natural gas must be mixed in just the right amounts with oxygen. With too much gas or oxygen, the flame will burn poorly or not at all. A yellow or orange flame means that too much gas and/or not enough oxygen is being used when burning. Another symptom is a black, sooty residue, as the gas does not burn cleanly.
Too Much Gas
One possible cause of the problem is too much gas. This can happen if the burner orifice is too large. This may be a result of wear on an old stove or due to a defect in the original manufacture. In either case, the orifice must be replaced.
Not Enough Air
Another possible cause is too little air. This may happen because the air shutter is improperly configured, because it is damaged or because it is blocked. This may be easier to fix than a problem with the burning orifice: a repair technician will clean the air shutter and adjust it to achieve the correct air-gas mixture.
Dealing with natural gas appliances such as furnaces and stoves is a tricky and dangerous business that should be done by a licensed professional. This problem in particular, since it can have a variety of causes that are difficult to diagnose individually, calls with special urgency for the service of a professional. While the furnace awaits repair, it should not be used.