Some of the best surviving examples of greco-roman use of colour are from Pompei. You can go and look at them yourselves, the Museo Archeologico in Napoli is a fantastic place to visit: https://www.museoarcheologiconapoli.it/en/portfolio-item/tem...

All the garish colours were prob heavily muted or diluted with varnish/oil. You don’t pant an artwork like a house, it is a layered technique and fairly similar to historic painting techniques used today:

https://emptyeasel.com/2014/12/02/how-to-paint-using-the-fle...

https://en.wikipedia.org/wiki/Grisaille