Should I dye only the roots?

My hair is dyed red but I haven’t dyed it in quite some time and my roots have grown out and they’re quite long. I don’t want to have to dye my entire head red again cos my hair is 35 inches and would take a lot of dye and I don’t have that type of money right now. Would it be bad if I dye from the roots of my hair down to the shoulders?? Before it was dyed red it was black and on the lower bits of my hair it's a bit darker and u can't rlly see the red. I don't want the top to be like super bright while the rest is dull. Btw it's not a super bright red, it's like a darkkkk cherry red.