A paint name or screen colour is not a reliable exact match. Primer, coat thickness, varnish, lighting and the paint’s opacity all change the appearance on the model.
Make a controlled comparison
Use spare material primed the same way as the original. Apply the candidate paint with the intended number of coats and let it dry fully. Add the same varnish before comparing under the lighting where the model will be displayed.
For a touch-up, start with the original paint and finish if you know them. For a new army scheme, choose a repeatable recipe and keep a labelled test piece. When mixing, record the paints and the measurement method; drops vary with bottle and viscosity, so an approximate drop recipe is not a laboratory colour formula.
Do not buy an entire replacement range on the strength of an online equivalence chart. Test one colour first, especially metallic, transparent and contrast-style paints. See Warhammer paint types and miniature brushes for application choices.