As Yarnabrina said before, if you want to specify a color you have to do it outside the aes() function and inside the desired geom_(), this is because you can't map an aesthetic to a character string (i.e. "blue"), see this example:
library(ggplot2)
ggplot(data=iris, aes(x=Sepal.Length, y=Sepal.Width, color='blue')) + geom_point(size=3)

ggplot(data=iris, aes(x=Sepal.Length, y=Sepal.Width)) + geom_point(size=3, color='blue')
