It's difficult to see what you are doing on the timeline when working with many layers and many keyframes. It would be really helpful to be able to add a colour highlight to whole layers and whole vertical strips of keyframes so you can see where you are more quickly when doing character animation.
That would be a great feature. However it doesn't exist. The closest that you can get is to use the coloring available to layers using the outline option for each layer. Another way to isolate layers is to organize your layers into folders and then collapse and open the folders to show only the section of the timeline that you want to work on.
There is no method for isolating or highlighting a frame or set of frames other than changing the size of the timeline window.
That would be a nice feature for sure! Very much like After Effects. I remember adding this to the wishlist years ago. Probably worth making your request count. Go here and add it to the teams' list: