Forming hidden patterns in porous substrates

A method for manufacturing patterned porous substrates with hidden color patterns by forming hydrophobic patterns on a hydrophilic surface, said method comprising the steps of;

• manufacturing structural channels in the form of a pattern in a porous substrate by flexo or gravure printing using a colorant-free hydrophobic printing solution on a front surface of the porous substrate, and
• applying a visible colored area on a rear surface of the porous substrate wherein the visible colored area is not visible through the porous substrate before applying a marking liquid, wherein the porous substrate is essentially opaque when in a dry state and wherein the porous substrate becomes at least partially transparent when wetted.