Purpose
Use this when you need a specific flat colour, for example a matching background. The field is the sRGB value you typed. It is not an HDR colour, not a brand-matched print colour, and not a calibrated patch. A shared link may put the hex in the address bar. That is a setting, not a finding. Query strings are marked noindex when the script runs. The canonical page is this one, without a query.
How to enter a colour
Use six digits, with or without a leading #. #F00 is rejected so the value cannot be ambiguous. The error sits under the box and the field does not change until the hex is valid.
Limitations
Opaque sRGB only. No alpha. A fragment or query can carry the colour you chose. It must not carry notes about a device. This page’s canonical address has neither.
Questions
Is this custom a calibrated reference?
No. It is an exact 8-bit sRGB field in the page. The display’s own settings change how it looks. It is not a white point, a meter reading or a profile.
Why is there a frame around the preview?
The frame is outside the field so the colour stays pure. Fullscreen removes the frame. If the browser will not enter fullscreen, the same colour still fills the view.
Can I save the field?
Yes. Download a PNG, or use the files on the downloads page. The file is a generated pattern, not a photo of your panel.
Does a mark on this colour prove a fault?
No. Compare it on white, black and the other primaries, and see whether a dry wipe moves it. Write down what you see.