Why the same hex color looks different on two screens

Join both screens to one session. Open the tool on the first screen and create a session, then enter the code on the second. Switch both to spectrum mode. Match brightness on the two screens first, turn off any automatic brightness or color changes, and give them a few minutes to settle. Keep the same distance, angle and room light for each screen. Then compare the field side by side. Look along the middle band, where the pure colors are, and at the far left and far right, which are both red. Check the top edge, which is white, and the bottom edge, which is black, for any grayish or tinted look on one side. Finally, look for any hue that appears flatter or duller on one screen.

Look for the kinds of differences you'd see in one patch, only spread across the whole range: a warmer or cooler tint, a hue that looks flatter, or a lighter or darker band in one part of the field. Those differences don't say which screen is wrong. Here's why they can happen. A hex color on a web page is an sRGB value, and CSS Color Level 4 says colors in HTML are treated as sRGB unless something else is specified. Screens don't all cover the same range of colors, though. The same spec defines an RGB color space's gamut by the chromaticities of its red, green and blue primaries and the chromaticity of its white point. MDN says the color-gamut media feature checks the range of colors a browser and its output device support. Settings like brightness, picture mode and color temperature change how the same color looks, and panel type matters for contrast and color consistency across the screen. So the same hex means the same numbers, not the same light.

The tool can't tell you why a difference exists, and it has no way to see the screens. It sends the same numbers and shows you the result, so the judgment is yours. Only a colorimeter can measure the light a screen produces, so use one if you need to know which screen matches the numbers. For a quick look, the spectrum field shows the differences across the whole range at once.