Get go2color for GO DAG plots.

def get_go2color_users(self,
                           usrgo_color='#feffa3', # yellow
                           hdrusrgo_color='#d4ffea', # green
                           hdrgo_color='#eee6f6'): # purple
        """Get go2color for GO DAG plots."""
        go2color = {}
        # Color user GO IDs
        for goid in self.usrgos:
            go2color[goid] = usrgo_color
        # Color header GO IDs. Headers which are also GO IDs get their own color.
        for goid_hdr in self.hdrgos_actual:
            go2color[goid_hdr] = hdrusrgo_color if goid_hdr in self.usrgos else hdrgo_color
        return go2color