The fundamental limitation of aerosol art has always been the inability to blend colors once they are airborne. For professional spray artists, this physics constraint translates into a logistical nightmare: hauling dozens or even hundreds of individual cans to a single worksite to achieve a full palette. James Provost, a former hardware engineer at Alphabet’s Google X, has spent seven years developing a mechatronic workaround called Spectrum—a system that mixes base colors while they are still pressurized, effectively turning four cans of paint into hundreds of custom hues.
Overcoming the Pressure Differential
The primary technical hurdle in mixing pressurized fluids is the inconsistency of internal can pressure. Provost’s initial prototypes utilized electronically controlled solenoid valves to release paint into a mixing channel. However, when two valves opened simultaneously, the paint from the higher-pressure can would flow backward into the lower-pressure can, causing the system to fail.
The solution was a shift in timing logic. Instead of simultaneous flow, Provost implemented sequential pulsing. By ensuring that only one valve is open at any given millisecond, he eliminated backflow entirely. The natural turbulence created by these rapid, sequential bursts is sufficient to mix the paints before they reach the spray head.
Technical Note: Pulse-Width Modulation (PWM)
Spectrum utilizes a concept borrowed from LED brightness control known as pulse-width modulation. By varying the “on” time (the pulse width) of each valve relative to a fixed cycle, the system controls the exact ratio of base colors. For example, a “clementine orange” is achieved by opening the yellow valve for one unit of time and the red valve for two.
Engineering a Clog-Proof Valve
Off-the-shelf solenoid valves, typically designed for water purifiers, proved useless for automotive-grade paint. The paint would enter the valve mechanism and harden, leading to constant clogging. These valves leaked the moment a can was disconnected due to the fact that they required constant inlet pressure to maintain a seal.
To solve this, Provost engineered a custom rotary pinch valve. This mechanism employs a stepper motor to rotate a lever with a rolling bearing, which constricts the flow within a flexible tube. Unlike standard valves, he added a spring to hold the lever in a closed position. This ensures the valve remains “normally closed,” isolating the paint can even when the motor is unpowered and preventing leaks during can swaps.
This custom hardware allows for extreme precision, with the ability to open for durations as short as 30 milliseconds.
The Control Stack and Hardware BOM
The brain of the Spectrum system is an Arduino Nano, which manages four knobs for color selection and a small display. The actual paint flow is triggered by a push button located above the spray head. Provost integrated a force sensor into this button, allowing the artist to create a color gradient; by increasing thumb pressure, the system shifts the mix from one pre-dialed color to another in real-time.
From a build perspective, the project is designed for accessibility. The fixtures are 3D-printed, and the total bill of materials (BOM) is under $150. The system uses four base colors—red, yellow, blue, and white—with pulse durations ranging from 30 to 250 milliseconds across eight possible values per color. While the mathematical combinations suggest over 4,000 possibilities, the actual number of distinct colors is lower because certain ratios (such as 2:3 and 4:6) produce identical results.
Analytical Q&A
Why not just apply a mixing chamber?
A mixing chamber would require simultaneous flow, which triggers the backflow problem caused by varying pressures between different spray cans.
What are the physical constraints of the tubing?
Provost determined that 1-millimeter tubing is the optimal balance; any narrower reduces spray force, while any wider allows paint to accumulate and settle in the lines.
Is this a commercial product?
No. It is currently a DIY project with project files and technical preprints available via the creator’s website and TechRxiv.
As the creator moves toward providing step-by-step instructions for non-technical users, will the democratization of this hardware significantly change the logistics of large-scale street art and mural production?
