Pick colours in OKLCH, see which ones fall outside sRGB or P3, and copy the CSS value or convert from hex and rgb.

A colour picker and converter for the OKLCH colour space, built by Andrey Sitnik and Roman Shamin at Evil Martians. You set Lightness, Chroma, Hue and Alpha with sliders, or paste a HEX, RGB or HSL value to convert it to OKLCH.
It can show the Display P3 and Rec2020 gamuts on its graphs and in a 3D model of the colour space that you can rotate and zoom. When a colour falls outside sRGB, it gives you the closest sRGB fallback by chroma.
The source is on GitHub under the MIT licence, and the page links to Evil Martians' article on why OKLCH works better than RGB and HSL.
oklch()