You can use products such as this:
This article is good too.
Article: Heat tint (temper) colours on stainless steel surfaces heated in air
It gives you an idea of the color vs the exhaust temp and explains why stainless turns color.
I've been on the road for 2 years now with my 445FE and the front of my pipes goes from blue to yellow.
FFR 6795, Delivered 10/26/08, 390FE Stroked to 447, Carb, T56, IRS, Pin Drive, 15" wheels, 3.55 gears