Semi-Permanent dye, just done?

I had like a dark brown coloured hair, i just dyed it for the first time.. I bought a darker shade of brown with some plum through it.. It came out, and it isn't noticable.. Well, it's darker and it's a little red at the top but i mean, not even that.. it's just, kinda the same.. only i can notice it? I got 6-8 washes.. On sunday i'm purchasing a deep red colour, can i dye over the top of it? It will be semi-permanent too. Is this okay? to dye over the top of it? (: x