The nave's walls were divided into two colored zones with a red stripe and two fillets (blue and white) at their edge. The lower zone has an umber stripe, that functions as a podea. The upper zone is a golden umber, indicating the background of Byzantine icons. The colouring was created with the unique background technique of Iannis Karoussos that was characterized as the trademark of his artwork.

The overall composition is based on Giotto's artwork, in the Basilica of San Francesco of Assisi, which was the prototype that Karoussos used for his work at the Great Cave Monastery in Kalavrita Greece. Specifically, the prototype refers to the Upper Church of San Francesco, where Giotto narrates the legend of St. Francis (1297-99). 

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The creative use of the architecture, together with colour structure, as a medium of achieving spatial reconstruction, are the key-elements of Karoussos's artwork. The fusion between figures and space, in his attempt to create a single coherent unity of 1.5-acre space, was his ultimate goal for his artwork at the cathedrals of St. Panteleimon in Athens, and St. Andrew in Patras (Greece). However, the spatial layout of the Great Cave narthex, does not include Byzantine architectural features, as it is a modern addition to the monastery complex. It concerned a single hall that the artist had to compose, to create a spatial aesthetic for the visitor, before entering  the cave.

To ensure the unity of the architecture and the iconographic ensemble, Karousos adopted the arrangement of the space of the lower zone of the Upper Church of St. Francis of Assisi, where Giotto included twenty-eight scenes from the life of the Saint. Each composition has a fictional (painted) cornice above the murals and each scene is framed by a painted colonnade. This fictitious structural depiction rendered an architectural completeness, with painting as a structural unit.

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The area of ​​the chapel of St. Spyridon is similarly composed, using the colour structure in the bi-zonal areas. The colour (umber and ochre) of the walls is integrated in the hybrid frescoes, creating a unified module between the architecture and the depicted space. Τhe paintings have a wood-like painted framing, as in Karoussos's artwork, however, the colonnade setting has not been included, due to the small scale of the church.

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