X-rays and ultraviolet images show that Mondrian removed existing paint when he added or repositioned lines or blocks of color. It wouldn't do just to paint over them. As he was shifting his lines around, he had to carve out space for them from thicker areas. These reworked surfaces and subsurfaces really reflect the impact that New York had on him at this point in his life. Conservation tools like X-rays can help show where and how Mondrian reworked his canvases.
Thicker paints absorb greater amounts of X-rays and thus appear lighter.
Areas where he scraped off paint and then reapplied it appear darker. Ultraviolet light can help identify pigments through fluorescence. For example, the cadmium yellow that Mondrian used in Europe fluoresces orange-red, but the yellow he used in New York does not. Fluorescence can indicate not only areas the artist changed, but also areas changed by others restoring the work. Mondrian's paintings have been subjected to invasive conservation treatments, which can cloud attempts to determine the artist's own changes.