x <- 1:100 
y <- seq(1, 300, 10) + rnorm(100)
y[5] <- y[5] + 10
y[10] <- y[10] + 5
y[25] <- y[25] - 10
fit <- lm(y ~ x)
colors=rainbow(10)
add_alpha <- function(color, alpha) {
  rgb_val <- col2rgb(color) / 255
  rgb(rgb_val[1, ], rgb_val[2, ], rgb_val[3, ], alpha = alpha)
}
alpha <- 0.65
rain <- sapply(colors, add_alpha, alpha = alpha)
plot(fit,col=rain)

x <- 1:50 
y <- seq(1, 300, 10) + rnorm(50)
y[5] <- y[5] + 10
y[10] <- y[10] + 5
y[25] <- y[25] - 10
fit <- lm(y ~ x)
plot(fit,col=rain)

x <- 1:120
y <- seq(1, 300, 10) + rnorm(120)
y[5] <- y[5] + 10
y[10] <- y[10] + 5
y[25] <- y[25] - 10
fit <- lm(y ~ x)
plot(fit,col=rainbow(500))

x <- 1:400
y <- seq(1, 300, 60) + rnorm(400)
y[5] <- y[5] + 10
y[10] <- y[10] + 5
y[25] <- y[25] - 10
fit <- lm(y ~ x)
plot(fit,col=rain)

x <- 1:900
y <- seq(1, 300, 30) + rnorm(900)
y[5] <- y[5] + 10
y[10] <- y[10] + 5
y[25] <- y[25] - 10
fit <- lm(y ~ x)
plot(fit,col=rain)

x <- 1:900
y <- seq(1, 300, 3000000000) + rnorm(900)
y[5] <- y[5] + 10
y[10] <- y[10] + 5
y[25] <- y[25] - 10
fit <- lm(y ~ x)
plot(fit,col=rain)