Fiber based product

A method for producing a fiber based product, the method comprising:

• a) applying a binder composition over at least one surface of at least one fiber based substrate to provide a treated at least one fiber based substrate, wherein the fiber based substrate is a veneer, a wood chip, paper, cardboard, or cotton, the binder composition prepared by using at least a polymerizable substance, a crosslinking agent, and a colorant comprising lignin, wherein the colorant is used in an amount such that the difference between a color of the binder composition and a color of a corresponding binder composition prepared without the use of the colorant, as measured on a Gardner scale, is at least 3 units;
• b) calculating the distribution of the applied binder composition over the entire at least one surface of the at least one fiber based substrate;
• and
• c) accepting the treated at least one fiber based substrate for the production of the fiber based product when contrast value is 50% or less, or rejecting the treated at least one fiber based substrate from the production of the fiber based product when the contrast value is greater than 50%, wherein the contrast value is defined as (Ds-Dt)/Ds*100%, wherein Ds is a reflection density of the binder composition, and Dt is a reflection density of the surface of the treated at least one fiber based substrate, as measured by reflection densitometry.