put colour inside the aes like this:
d<-data.frame(x=1:5, y1=1:5, y2=2:6)
ggplot(d, aes(x)) +
geom_line(aes(y=y1, colour="1")) +
geom_line(aes(y=y2, colour="2")) +
scale_colour_manual(values=c("red", "blue"))
but I recommend this way:
d2 <- melt(d, id="x")
ggplot(d2, aes(x, value, colour=variable)) +
geom_line() +
scale_colour_manual(values=c("red", "blue"))