Helene remains tirelessly committed to opening up her collection for as large an audience as possible. In 1923 she organizes a series of lectures for the Volksuniversiteit Den Haag in the exhibition spaces at Lange Voorhout. In 6 lectures she examines the development in art from realism to abstraction since circa 1850 on the basis of her collection. Inspired by Bremmer's ideas, Helene developed her own theory about modern art and like him, prefers to speak of realism and idealism. The realists capture the visible reality, with attention paid to light, colour effects and perspective, while the idealists depict their ‘idea’ of reality. In order to demonstrate the differences, she always juxtaposes two works, a method that often used in his lessons as well.

Helene regards George Breitner, Henri Fantin-Latour, Paul Gabriel, Isaac Israels, Jean-François Millet, Auguste Renoir and Jan Weissenbruch as realists. The true idealists are Auguste Herbin, Juan Gris, Bart van der Leck, Piet Mondriaan and Pablo Picasso. To illustrate the development from realism to idealism, she shows work by Odilon Redon, George Seurat, Paul Signac and Jan Toorop. Helene’s favourite Vincent van Gogh occupies a special position. He is ‘one of the great spirits in modern art’ and defies any categorization.

To show that the distinction between realism and idealism is something of all ages, whereby the two movements alternate, she also supplements her collection of old masters. She acquires the large work Venus with Amor the honey thief by Lucas Cranach the Elder and Portrait of a man by Jacopo Robusti Tintoretto. In this way, the Chinese porcelain and earthenware that she collected much earlier also acquires a theoretical place in her collection.

The enthusiasm for the lectures is so great that Helene decides to publish her lectures. Considerations regarding problems in the development of Modern painting is self-published in 1925. The emblem on the cover, a design of Henry van de Velde, is a representation of Helene’s motto ‘Spiritus et Materia Unum’.

It is the last commission that Van de Velde completes for the Kröllers. In 1926 he gives up his employment for a professorship at the University of Ghent and becomes director of the recently opened Institut Supérieur des Arts Décoratifs de la Cambre in Brussels.

After lending part of her collection to the Sonderbund exhibition in Cologne in 1912, Helene again responded to a request in 1927 to loan her Van Gogh collection. 140 drawings and paintings from her collection are included in a large retrospective that is shown in Basel, Bern and Brussels. The exhibition attracts 30,000 visitors in Brussels alone. As of August 1928, the exhibition, complemented with 400 works by other artists from Helene’s collection, travels on to Düsseldorf, Karlsruhe, Berlin and Hamburg. 

While the Van Gogh collection is travelling through Switzerland, Belgium and Germany, Helene organizes solo exhibitions in her museum at Lange Voorhout, with Bart van der Leck, Odilon Redon and, on the occasion of his death, Jan Toorop. She also presents double exhibitions, including that of Joseph Mendes Da Costa and Dirk Nijland.

When the Van Gogh collection returns in April 1929, she concludes the series with a major retrospective in which her 98 paintings, 24 aquarelles and 135 drawings by the artist are shown. The exhibition attracts visitors from all over the world.