Does the DP has to be used pure or can it be mixed with distilled water or/and water colorant like glow motion?

Hi! Last year during Winter if I shutdown my computer for too long the liquid of my water circuit would froze because my radiator is located outside my flat. Is there an additive that I can be added to the distilled water + anticorrosion additive that would make it freeze resistant or lower the chances of it? If yes what and in which proportion? Thanks ^_~