Your exhaust is made in stainless, not titanium. The color change is because the temperature difference in the header pipe to the tail pipe. The header gets hot enough to glow red and over time the surface oxidises slightly. The tail pipe doesn’t get anywhere near as hot to oxidise. Nothing to worry about.
Tell us more about your bike. How many owners did it have? How many miles are on it? Did it come with a key code card? How many keys? Any service records? Or is it a mystery-bike, like the ones I’ve been discovering?