has flowed up to the position marked solvent front.

That is the so-called solvent front, and then you can see the result.

Chlorophyll a travels the furthest distance (closest to the solvent front) and is blue-green in colour.

Substances with very low solubility in n-octanol tend to give abnormally low log Pow values with the HPLC method; the peaks of such compounds sometimes accompany the solvent front.