Illumi ring is useless to Sinys, red seems okay to a degree. Although green seems to work well, but i believe he will learn something similar later? So if he doesn't learn such a thing I'm liking green more. Otherwise red is smartest choice.
@Fallenreaper
But inbetween sento and may?
Another thing you might want to consider, using a ring prevents him from using Mage Blood. You use runes, it has its own source rather than tap into yours and so won't cause you sickness.
1x Thank
Hehee 
