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As we can see on this picture from Mary "@bocachicagal" on NASASpaceFlight, there are two types of tank near the test stand.

Some are black and others are white or bright and shiny metal, but why?

SN5 on test stand

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  • $\begingroup$ The silvery tank is a test Starship tank undergoing cryogenic or pressure testing. The white tanks probably hold cryogenic LOX, LN or LNG. The black tanks may be for the water deluge system and/or parts of the system that hold a buffer of excess boil off methane before compression cooling and liquefaction. $\endgroup$
    – Slarty
    Apr 7 at 8:14

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Normally cryo tanks are painted white to reduce the heat absorption.

Maybe they paint them black because they want to increase the pressure inside them.

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    $\begingroup$ If they need an increased but constant pressure they would not rely on sunshine. $\endgroup$
    – Uwe
    Nov 12, 2020 at 19:43
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    $\begingroup$ Polished metal in sunlight can get hot enough to raise blisters on your skin from touching it. Sunlight in Texas is bright enough to do it. $\endgroup$
    – JRE
    Nov 13, 2020 at 7:59
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    $\begingroup$ @uhoh It's much easier to get a very reflective white paint than very reflective mirror polish. A common mirror has reflectance of ~80% while a good common white paint will exceed 90%. Steel tanks won't be better than mirrors. $\endgroup$
    – SF.
    Dec 13, 2020 at 11:06
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    $\begingroup$ If you get into 'ultra', like 99.99% like in high energy laser optics, dedicated mirrors will outperform paint, but % of reflection per dollar, white paint wins hands down. $\endgroup$
    – SF.
    Dec 13, 2020 at 11:10
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    $\begingroup$ @JRE I've never gotten metal blisters in Texas but I have noticed and been perplexed by shiny metal getting uncomfortably hot in the Sun. I suppose it's related not only to less-than-mirror-like metal surfaces absorbing 10-30% of incident power, but also metal's high thermal conductivity's ability to promptly flow so much of the collected heat back into a relatively cool finger in contact with it. $\endgroup$
    – uhoh
    Dec 13, 2020 at 22:42

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