To modify the color of main title consider changing plot.title via an element_text() call, as demonstrated in this reproducible example:
library(ggplot2)
ggplot(data = pressure, aes(x = temperature, y = pressure)) +
geom_line(color = "red") +
labs(title = "It's a bird? It's a plane? It's a ggplot chart!") +
theme(plot.title = element_text(color = "limegreen"))
