I see. This is quite a tricky situation I've brought unto myself, then. Perhaps we can use "palely coloured"? Since I believe creating new verbs is possible by attaching -lat, would visshiyalat more or less fit the job?
Vidrogerat lamees fin nem visshiyae dei, "to ride the mare that is lightly coloured."
Vidrogerat lamees fin nem visshiyae dei, "to ride the mare that is lightly coloured."