I had that idea once, but half way does not work well, since half way between UTC+3 and UTC+4 is UTC+3:30, but this has nothing to do with DST. So even if half way is used one needs to alter another parameter apart from hue. The model is HSV that leaves S and V for DST markation. On the Indiana map I wanted to have different colors for two hues, the map only uses UTC-6 (hue=0 ~ red) and UTC-5 (hue=60 ~ yellow), see commons:File:UTC TZ hue4map USA-IN.svg.