hdri has 32 bits per channel, whereas ordinary jpeg has 8 bits per channel (RGB).  hence in poser, hdri is much brighter light source than jpeg texture, with greater range between darkest and lightest colours.  to boost brightness of simple colour, increase value of light-emitting channel. simple colours burn out in poser GC renders if value is too high, but some colours won't burn out in some HSVTM poser renders.  boosting value of jpeg will make it brighter in poser, but it won't have wide range of brightness.