I use Sunset Amnesia for the butt section of my nymph leaders. This is a brightly colored monofilament line intended for use as a running line, but also frequently used for leader butt sections. Just Google "Sunset Amnesia" and you should find a variety of sources.
Another approach would be to use a "backing sighter". You can make one from some braided backing line, see for example https://www.youtube.com/watch?v=giH3O-jYkgQ. If you don't have colored backing, use white backing and color it with a fabric marker.
One other thing that might be helpful is S'getti String". This is a small diameter tubing that comes in bright colors and that you can find in craft stores. Just cut a short piece and slide your leader through it. Your leader knot will hold it in place.
There are a variety of other "sighters" that are used for European / Czech nymphing. You can buy them pre-made or make your own.