Brown is the most common eye color and it is the one most often badly served, because the default advice is brown shadow — and brown shadow on brown eyes is the one combination guaranteed to disappear. There is no contrast between the lid and the iris, so the work you did does not read from more than a foot away.
The color principle is contrast, not matching. A dark brown iris is deepened by something adjacent to it rather than identical to it. Cool plums, soft mauves, rose-browns and taupes all sit far enough from a warm brown iris to make the eye look more defined; warm coppers and bronzes do the same thing from the other direction by making the brown read richer. A flat mid-brown does neither, which is why so many people conclude eyeshadow does nothing for them.
But over 50 the finish matters more than the color does. This is the part the eye-color advice never mentions. A crepey or finely lined lid has texture, and shimmer particles sit in that texture rather than across it — so a shimmer shadow that looked luminous at forty reads as glitter collected in lines at sixty-five. Matte and satin finishes diffuse evenly. One satin shade as a highlight is fine; a full shimmer lid is not.
And the placement question is decided by the lid, not the iris. If your lid has become hooded, color placed in the natural crease is folded away and invisible when your eyes are open, so it has to go slightly above the crease where it can actually be seen. That is a separate decision from shade and it is covered properly on the hooded eyes page — read that alongside this one if your lids have changed shape.
One practical note that outranks all of the above: on a mature lid, shadow does not stay where you put it without a base. Oil and movement migrate powder into the crease within a couple of hours, and no amount of correct shade selection survives that. A thin eyeshadow base is the difference between this working and not — see best eyeshadow primer for mature eyes.
