I used the legend() function.
set.seed(1)
DF <- data.frame(A = rnorm(20), B = rnorm(20, 5, 1))
hist(DF$A, col ="red", xlim = c(-2, 8), breaks = seq(-3, 7, 0.5))
hist(DF$B, col ="green", add = TRUE, breaks = seq(-2, 7, 0.5))
legend(x = "topleft", legend = c("Pop A", "Pop B"), fill = c("red", "green"))
Created on 2020-09-16 by the reprex package (v0.3.0)