Bluing for the first 15 inches or so is normal when the engine is tuned to make power and is ridden hard.  The color is an indication that the wall of the pipe has reached a certain temperature.  There is a direct correlation between color and temperature.  It has been many years since I looked at the tables of color vs temperature but I think that yellowing occurs in the 500 to 600 deg. F range, bluing occurs when the plating reaches 700 to 800 deg, and purple occurs when the surface has been over 900 deg. F.

The exhaust temperature on most well developed two strokes will be in the 1125 to 1250 deg. F range at wide open throttle.   The temperature of the wall of the pipe depends upon the exhaust temperature inside the pipe, the wall thickness, the thickness of the plating, the velocity of the air moving over the surface of the pipe and how long the engine RPM and throttle position have been maintained.

The type K thermocouples with the 3/16" diameter probes are the most common probes sold.  The 3/16” probe is durable but responds too slowly to follow the true exhaust temperature for drag racing and most types of riding.  It has been my experience that a 3/16” probe on an rapidly accelerating engine (around 1.5 sec. per gear of less) will often indicate a temperature of 1050 deg. F when a small fast response time thermocouple will indicate 1200 deg. F.  This is one the main reason why many guys are burning pistons when their EGT gages are reading under 1050 F.