Sounds like you're talking about vector drawings like Inkscape and the svg format. PNG is mostly used in raster images which include a list of hex values. To minimise the complexity of the image (as mentioned above) the programming will first simplify the image, sometimes by simply averaging similar values over an area of the image then gets fed to a compression routine that substitutes those averaged values with symbols(I.e ten 'x's become "x10" or something similar). SVG and vector images have the same hex, describing line segments, curves and colours.