It's an art history/architecture joke.
For a long time, it was believed that the famous Greek/Roman statues that you see in museums were supposed to just be unpainted marble, but within the past sixty years or so, archeologists have discovered microscopic flecks of paint on the statues and realized that they were actually painted and the paint just came off in the intervening couple thousand years--or was intentionally rubbed off by archeologists and curators of centuries past, for the sake of making them "look right". And, as is described in the tweet pictured above, people have managed to analyze what little bits of paint remain and determine what some of these statues were supposed to look like. They were actually painted with bright colours and patterns.
Brutalism is a 20th-century style of architecture which eschews decoration in favour of bare, unpainted brick or concrete. The joke is in suggesting that the buildings aren't really supposed to look like that, the way that the ancient statues aren't supposed to look like that, even though all Brutalist buildings were built within living memory rather than thousands of years ago. (And it's possibly also poking fun at Brutalism as a style, for being so intentionally austere.) The buildings in the above photos have been edited to match the statues, with bright colours and patterns and Ancient Greek-style murals.
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