After dying my hair, my roots are a different color than the rest of my hair. How do I fix it?

I dyed my hair black about a month and a half ago. I tried to dye it dark brown lat night, but my roots came out light brown and the rest is still black. Is there any way I can fix this other than dying my hair black again? I like the black but it's a little too harsh for a while. Anything would help!