on the top or the timeline where caching of the frames is shown (with grey rectangles) there could be 2 colored markers/rectangles for start/end that you see in animation docker. When you change the values in the docker,they should be updated in the timeline. Potentially just using the TINT color from the onion skins could suffice,but if this needs to be seperate , seperate colors are fine too. if the value is the same ,maybe show a rectangle with both colors,cut in half?
It needs a simple GUI mockup that users agree on (something not distracting, but clear) before it can be implemented.
Starting in Krita 5.0, the frames outside of the playback range will be shown slightly darker than frames inside the playback range. That should solve this for now, but if people feel there needs to be more done here, we can reopen this.