A method of producing coloured portions on a tyre, the method including the steps of: producing a highly impermeable protective layer by depositing a water-based emulsion on a tyre part to be coloured; and applying a water-based paint to the protective layer. The water-based emulsion has at least a cross- linkable polymer base, and a surface-active agent of molecular formula (I) (R1CONR2CHR3COO−)n Xn+(I) where: R1 is an aliphatic group C6-C23, R2 is H or an aliphatic group C1-C8, R3 is H or an aliphatic or aromatic group C1-C8, X is a metal cation, preferably an alkaline cation, and n is an integer of 1 to 3.