Why is my hair so greasy after i wash it?

For the past couple months my hair has been super greasy and my mom tells me not to wash it every other day but it's DISGUSTING!!!! I try not to wash it as often as I can because i semi-permanent dye my hair and I dont want the color to come out...I've been using Nexxus shampoo and i recently stopped using the conditioner and sometimes it actually dries greasy..and I'm 16. I've also tried baby powder and that doesnt work...I've also tried dry shampoo sprays and that doesnt work either.. and I've also tried using cold water before i get outta the shower but it doesnt seem to help...What can I do so i dont hafta wash my hair so my color washes out.please help :( Thanks<3