Ordered 2-D structures composed of poly(N-isopropylacrylamide) (PNIPAM) microgel particles that had regularity on a sub-micrometer length scale were prepared. By using sterically stabilized PNIPAM microgel particles as components, the ordered array was formed by a self-assembly process. The particle array was prepared by depositing a droplet of the microgel dispersion on a substrate. Atomic force microscopy observation of the resulting thin films revealed that they comprised a monolayer particle array. The periodic structure of the array produced iridescent colors due to optical diffraction. Since a homogeneous particle array can be prepared simply by drying the dispersion, this particle dispersion may be considered as a new ink whose color is generated from the microstructure in the films produced.