ggplot2: adding more lines in a spesific color

I’m trying to add more lines to my line chart. How do I ensure that all newly added lines are greyed out?

first4letters <- c(rep("A", 3), rep("B", 3), rep("C", 3), rep("D", 3))
times <- rep(c("time1", "time2", "time3"), 4)
score <- c(5, 10, 2, 3, 4, 7, 11, 2, 9, 4, 6, 11)

df <- data.frame(first4letters, times, score)
df

plot1 <- ggplot(data = df, aes(times, score, group = first4letters)) + 
  geom_line(aes(color = first4letters)) +
  geom_point(aes(color = factor(first4letters)), size = 1) +
  theme_classic()
plot1 + scale_color_manual(values = c("red", "blue", "black", "orange")) 

I will add some new letters. However, the lines showing these letters must be gray. I cannot add a new value to the scale_color_manual() function. Because the function determines the order of the colors itself.

>Solution :

One option would be to use a named vector of colors. Doing so, all other categories are automatically assigned the na.value=:

Adding two more letters to your example data:

library(ggplot2)

first4letters <- c(
  rep("A", 3), rep("B", 3),
  rep("C", 3), rep("D", 3),
  rep("E", 3), rep("F", 3)
)
times <- rep(
  c("time1", "time2", "time3"), 6
)
score <- c(
  5, 10, 2, 3, 4, 7, 11, 2, 9, 4,
  6, 11, 3, 8, 9, 10, 12, 3
)

df <- data.frame(first4letters, times, score)

pal_color <- c("red", "blue", "black", "orange")
names(pal_color) <- LETTERS[1:4]

ggplot(data = df, aes(times, score, group = first4letters)) +
  geom_line(aes(color = first4letters)) +
  geom_point(aes(color = factor(first4letters)), size = 1) +
  theme_classic() +
  scale_color_manual(values = pal_color, na.value = "grey80")