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The "blue" is a result of the thickiness of the oxide ayer that grows with heat, and is more a rainbow of colours.
The oxide grows faster at higher temperature, but you can reach he same oxide thickness, and same colour, at lower temperature with longer time.

Is the same process as "temper colours" for steel, except in that case it is iron oxide, not chrome oxide.
Also same process as the rainbow colours on titanium.

what causes the colour is "thin-film interference", due to the different lengths of the light paths of the light that bounces off the outside of the oxide film, compared to the light that bounces off the oxide/metal interface.

there are different orderes of interference, and for some materials you can get multiple rainbows. (eg SiO2)

for the exhaust above, starting at the springs and going toward the head, you can see the same progression fo colour as in the image below.
ie, at springs, approx 400 on the X-axis, and at the head is around 1000.
for chrome oxide, above that oxide thickness it turns black, rather than going through the next order (and next rainbow)

Rectangle Font Slope Magenta Parallel


Rectangle Slope Font Parallel Handwriting
 

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and.. if the headers are titanum, they are much more likely to get funky temper colours without getting stupid hot, compared to getting same colours on stainless steel (which most comments on internets about colour and heat are probably related to)

Aren't they titanium?

what is really good, is that each of your cylinders is changing the pipe colour at the same rate.. so at least they are all even :)

If you get bored, or change tuning, you can polish them back to bare Ti metal, and watch them colour up again.

some nice images of new vs coloured Ti headers on this site
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