Bright exhaust in afterburner vs no visible (bright) exhaust when not in afterburner?
It's because on a jet engine the flame in the burner can is carefully controlled and contained by airflow that is guided in from the sides to keep the flame in the center of the burner can so it doesn't touch metal (only maybe a 3rd to half of the airflow out of the compressor gets burned; the rest is for temperature control - which is why there is enough oxygen in the air leaving the turbine to support reheat).  The flame is supposed to be out before the gasses have left the burner can or else the turbine will overheat. The turbine itself is getting burned air that has been mixed with un-burned air to cool it off.  

On most engines, if you see a light in the tailpipe of an engine not in reheat, it means there is a flame aft of the burner can in the turbine zone, which is very very bad, except when initially starting and some is briefly visible after lightup. 

On some engines, if there is a direct visual path up the tail pipe, through the turbine and stator blades, into the center of the burner can, you may be able to make out a bit of bluish light (should be bluish; yellow light means too rich and the yellow is un-burned carbon fluorescing, like any yellow flame).