Set specific colours on line using ggplot

Question

I have created the following graph that contains multiple lines:

set.seed(1)
df_test <- data.frame( x = c(rnorm(100, 0, 3), rnorm(100, 0, 10)),g = gl(2, 100))
ggplot(    df_test, aes(x, colour = g)) + stat_ecdf()

I would like to change the colours of the lines (e.g. black and blue). Any suggestions please?

K