You can use scale_color_manual() in the same way I have used scale_fill_manual().
DF <- data.frame(Species=c("Emperor","Cape","Hoboken"),
Value=c(23,34,19))
library(ggplot2)
COLORS <- c(Emperor="red",Cape="blue",Hoboken="green")
ggplot(DF,aes(x=Species,y=Value,fill=Species))+
geom_col()+scale_fill_manual(values=COLORS)

Created on 2022-02-02 by the reprex package (v2.0.1)