The Marker or Stop knot
Take your chosen marker material, I recommend Fox Magic Marker Braid as it does exactly what it says on the tin and is purpose made for this job. make a rough loop with one end longer than the other and lay it against your main line ( shown in green)
Whilst keeping pressure on the main line ( in green) moisten the knot and pull gently tight by pulling the two tag ends of the red line. Be careful not to trap the mainline as if you do whilst tightening it you may kink and weaken the main line
And there you have it a knot that can be used as a marker knot or stop knot.