...From that fact we can deduce that modernists have been rationalizing the use of colours.

On the contrary, can we say that “Design in the White Cube” posters go against this rationalization of the use of colour? On the one hand the colours are indeed still vivid/saturated, but on the other hand it uses a slightly richer set of colours (five instead of two: green, red, purple, black, and blue). Luna Maurer’s project “Living Agenda” can be seen to support this: the artist informally declared during the presentation she gave to media design students of the Piet Zwart Instute in 2008 that she decided to order tape from USA because the range of available colours there was larger. She could have used primary colours whicher were easily available on the local market, as most modernists would probably have done. But contrast to modernists who were reducing the number of colours, she was looking to increasing them.