SAN FRANCISCO — Lumio, a San Francisco–based technology startup founded eighteen months ago, announced Tuesday that it has closed a $40 million Series B funding round for its flagship product, Lumio Tap, a mobile application that sends payment reminders to friends and is in all measurable respects identical to existing payment-reminder application Greenlight, except that Lumio Tap's primary interface color is described in the company's pitch deck as "Horizon Cerulean."

The round was led by venture capital firm Parallax Ventures, whose managing partner Brice Holloway called the product "a bold reimagining of the button."

"What Lumio has done here," Holloway told reporters at a Tuesday press event held in a warehouse that contained one large television and seventeen Aeron chairs, "is fundamentally challenge our assumptions about what a button can be, where a button can go, and, frankly, who gets to decide what color a button is."

Lumio Tap allows users to tap a button to send a payment reminder. Greenlight, which has been available since 2019, allows users to tap a button to send a payment reminder. Independent testing by the University of Delaware's Center for Redundant Interface Studies found no functional difference between the two applications across 240 trials, though researchers noted that Lumio Tap's button "does produce a slightly more satisfying haptic click, which is not nothing."

"We're not just building an app. We're building a philosophy of transactional nudging that happens, yes, to also be an app." — Dex Farrow, Chief Reimagination Officer, Lumio

Lumio CEO and co-founder Dex Farrow, who previously held the title of Chief Friction Disassembler at a startup that sold bluetooth-enabled napkin holders, said the company's vision extends far beyond color.

"Greenlight is green," Farrow said, pausing for effect. "We asked ourselves: does it have to be? The answer we arrived at, after fourteen months of design sprints and a silent retreat in Ojai, was: no."

The $40 million will be allocated across three areas: $18 million in engineering to "rebuild the infrastructure from scratch in a way that produces the same output," $14 million in marketing to explain the difference to users who have not yet perceived one, and $8 million in what the company's term sheet describes only as "vibe capital."

Greenlight, reached for comment, issued a statement noting that its app is also available in a dark mode, which is technically closer to blue.

Lumio Tap is expected to launch in public beta next spring, at which point the company anticipates beginning work on Lumio Tap 2.0, which sources familiar with the matter say will introduce a rounded corner on the button that is, by internal measurements, "meaningfully more rounded."

The app is free to download. Sending a reminder costs $1.99.