We have designed and successfully prepared polymer membranes as manageable films for colorimetric sensor materials for the visual detection of the acidity of water in the low pH range, below 4, and beyond the pH scale, up to a concentration of 11.6 M of perchloric acid. For this purpose, two monomers with weakly basic groups were synthesised, with these monomers containing up to four protonisable groups in the aforementioned acidic regions. The pKa values were calculated for the membranes in water, which exhibited gel behaviour, and ranged between 2.7 and −6.5. Accordingly, visual colour changes qualitatively indicated the acidity of water containing perchloric acid, and the quantification was performed inexpensively by UV/Vis spectroscopy. Titration with other acids is foreseen for practical applications. Moreover, polymer coatings of conventional cotton and high-tech p-aramid fibres, using the designed sensor monomers, yielded smart fabrics, which responded colorimetrically to the acidity of the water media. Furthermore, both membranes and smart fibres exhibited colour changes in air under acidic atmospheres, demonstrating the future development of fully sensory apparels and smart tags with envisaged health and safety applications. In addition, the sensor materials are reusable, because the protonation under acidic conditions is reverted by washing with water, and are highly stable under acidic conditions for long periods of time.