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Poly(styrene-co-N-vinylformamide) (poly(St-co-VFA)) nanoparticles with the diameter from 55 to 200 nm were prepared by emulsion copolymerization and their self-assembling was studied. In the dispersions of these particles, the formation of iridescent periodic colloid structures is observed. The period of the structures is about 220 nm and shows no cymbate dependence on the nanoparticle size, indicating the possible variety of packing types occurring during the self-assembling of nanoparticles in dispersions. For 250-300 nm sized poly(styrene-co-methacrylic acid) (poly(St-co-MAA)) particles, it was shown that ionization of their surface carboxylic groups in alkaline media leads to the considerable improvement of the photonic crystal structures obtained from the dispersions of these nanoparticles. It was demonstrated that cationic chromophores can be adsorbed on their negatively charged surface. In this way, chromophore-modified polymer photonic crystal materials for applications as gas or liquid sensors could be prepared, as their measured fluorescence spectra show remarkable and reversible responses to the presence of polar organic solvent vapors, e.g. acetone.