Aesthetics Must Be Either Length 1 Or The Same As The Data

How to resolve the Error: Aesthetics must be either length 1 or the same as the data (2): fill?

I have a code like

> b <- ggplot(data= yield2016_drought7,aes_string(x = "Condition", y = "FT"))

> b+ geom_boxplot(notch = T, fill = c("yellow", "orange", "red", "pink"), outlier.color = NULL) + ggtitle("Flowering Time") + theme(legend.position = "none") + theme_classic()

Error: Aesthetics must be either length 1 or the same as the data (1): fill