The synesthesia works explore the sensorial relationship between color and abstract concepts of time and space. In collaboration with American artist and filmmaker Lucy Cordes Engelman, whom he is married to, Mullen paints as a conduit for her sensorial experience between color and time. Each color they use represents a number, and when these numbers are used to illustrate time the painting appears as a layered image, a representation of time. Mullen's relationship with this universe began just after he approached Lucy, for whom numbers and letters connect to colors in a different way. She currently collaborates in the production of the aforementioned series, assisting him on the definition of chromatic matches, through brush strokes in the shape of lines in his paintings. The couple's creative process begins by choosing specific dates. Each date turns into a mathematical fraction which results in specific color in Lucy's brain. Number two, for instance, is a shade of yellow. "She unveils the codes of time, discovering each number's colors through her very own eyes. Time and color are the two necessary varying factors she needs to unravel the equation", explains the artist.
The precise match between pigmentation and geometry present in the synesthesia's set creates unusual perspectives, and confuses the viewer with all sorts of optical illusion, in a direct reference to the kinetic movement in the 50s. Canvases gain volume throughout the artist's technique; it appears to look like an encrustation of dozens of multicolored glass plates that move toward the observer.
For the synesthesia works in Shifting Perception the numbers chosen to be represented were chosen by the couple because they were interested in individual canvases creating an interaction together. Choosing odd numbers next to each other (five, seven, and nine) gave an opportunity to explore the different colors in a way never before. This creates involuntary relationships between specific colors and numbers.