Pick a foreground and background color and see, before you print anything, whether the contrast between them is strong enough for a phone camera to read reliably.
Illustrative preview — not a decodable QR code. Generate an actual code on the QR generator.
This tool borrows the WCAG relative-luminance contrast formula — the same math used to check whether text is readable against a background — as a practical stand-in for barcode scannability, since both problems come down to the same thing: can a sensor reliably tell two colors apart. A ratio of 4.5:1 or higher is a safe target for most printed QR codes; between 2.5 and 4.5 is workable but risky in poor lighting, on a small screen, or after printing softens the edges; below 2.5 is likely to cause failed or slow scans.
Most QR decoders — including phone cameras — are tuned to expect dark modules on a light background, since that's the overwhelming majority of real-world codes. An inverted combination (light modules on a dark background) can still pass a strong contrast ratio and may scan fine on modern phones, but it's worth testing on a couple of actual devices before committing to a print run, especially if the code will also be scanned by older or dedicated hardware scanners.
A good ratio on screen doesn't guarantee a good result on paper. Low-end inkjet and thermal printers can shift color saturation, and glossy or textured stock can introduce glare that effectively lowers contrast in certain lighting. See our printing guide for the other factors — sizing, quiet zone, file format — that combine with contrast to determine whether a code actually scans once it's out of the browser.
Happy with your colors? Build the real, scannable code on the QR code generator, which uses the same foreground and background fields.