I swear, I thought I've seen folks mention that the OEM tanks turn a brownish color with age as an indication that they may be becoming brittle...I’ve never heard that one before. Expansion tanks don’t tend to fail catastrophically, they develop hairline cranks or pinholes which you first notice because there’s a small puddle of water under the front corner of the car after a run. Unless you’re doing preventative maintenance before a big road trip, I would leave well alone.
Thanks for that info! Good to know.All the hard plastic cooling stuff is that color, I think it's just the material type. The coolant hard pipes underneath the intake manifold are also the same brown under a flashlight but appears black to the naked eye.