What is the principal pigment of plant?
Chlorophyll a and b are the principal pigments of plants.
What is the relationship between the solubility of the plant pigment and the distance it ravels on the chromatography paper?
Plants are green because of the presence of a green pigment called chrolophyll. This pigment is responsible for sunlight hervesting. When you make a plant extract this pigment get solubilized into extract by the phospholipids (a major component of cell wall) leading to green color. Actually, most plant extracts are not green, and you can see the colors at herbalextractsupplier.com.