Color only really shows on the roots?

Hey guys so I have a question. I dyed my hair a few months back with a demi permanent in a medium auburn tone. My roots were pretty red but it faded gradually as it went down the rest of my hair. I'm not sure why that is exactly. My natural hair is a medium golden brown with a red hue to it. My sister also dyed her hair a demi permanent lighter auburn and only the roots really showed the color we wanted to achieve. Is there a reason why it does that? Her hair color is the same as mine though she has a bit of a blonder hue.

Obviously I don't expect the hair color to change THAT much since it's only a demi but I'm still wondering why only the roots really show the color. Should we leave it on longer last time? They were different brands but the same principal I would assume.

I used: Garnier Herbashine Haircolor in 565 Medium Auburn Brown

My sister used: loreal casting creme gloss in 645 amber