Correct SimonH.
It is the natural discolouration from heating.
Stainless exhausts do it on bikes and M3W.
It can be polished out.
The colours used to be referred to as "temper colours" by blacksmiths etc. who would heat items to the desired colour before quenching to attain the required final metal hardness/temper state.
It is also used creatively to form coloured surfaces.
Titanium is often heated to generate colours for cosmetic use. The Pagani has such a purple coloured resonator box to tune the exhaust note....baked to attain a specific colour.
To leave such discolouration visible on a premium car is not good presentation or image.
Also, the filler metal used needs to be correctly chosen...."weld decay" is a term used where a filler of stainless that has too high a carbon content is used. The carbon combines with chromium to form carbides which weaken the weld.
Use of low carbon and titanium stabilised filler metal combats it.
