Whats the difference between semi and demi-permanent hair color?

Im trying to find something that will last longer for when i dye my boyfriends hair.Whats the difference between semi and demi-permanent hair color?
Demi-permanent lasts longer than semi-permanent. Usually fades after 18-20 washes while semi-permanent begins to fade after 4-6. Darker colors can last longer though, so be careful.Whats the difference between semi and demi-permanent hair color?
They are basically the same. Depending on the manufacturer the demi might last a little longer. If you really want it to last, just use permanent
ones permanent and wont come out and one washes out in somthing like 10 washes