You mix the right grey on the palette, paint a repair and return later to find a visible patch. The wet mixture was only one stage of the colour. A dried sample on the intended surface is a more useful reference for matching it.
Acrylic formulations and mediums differ. Some mixtures change appearance as they dry, and the support underneath also affects what you see. GOLDEN’s Glazing Liquid guidance, for example, notes that colours mixed with that product appear weaker and lighter while wet. That is a product-specific reason to test, not a claim that every acrylic changes by the same amount.
Build a swatch you can reproduce
Use a spare piece prepared like your painting surface. Record the paint names, approximate mixture proportions and any medium. Paint a thin area and a more opaque area if both will occur in the work. Leave enough unpainted space to label the sample after it dries.
Compare the dry patch with the target under the light in which the finished piece will usually be viewed. Keep both at the same angle. A glossy patch beside a matte area can look different because of reflections even when the pigment mixture is close.
Try one adjustment, then wait again
Suppose the first dried grey looks too dark. Mix a second, slightly lighter sample while leaving the original visible. Do not repeatedly repaint the first square; you will lose the comparison and change the thickness at the same time. Number your samples and write the mixture beside each one.
For a large piece, prepare enough of the successful mixture for the area where continuity matters, following the paint’s storage guidance. If you must mix again, the dry sample gives you something concrete to match.
Keep varnish tests separate. A final coating can alter sheen and appearance, and it needs its own compatible test after the paint has dried as recommended. A little labelled sample board is less exciting than the finished painting, but it can prevent a small correction from becoming a much larger repaint.