in the bellow change "a", "b" "c" etc, to the names of your instruments, and relate them to what colour you want. Then use scale_fill_manual as shown.
list_of_colours_per_instrument <- list(
"a"="blue",
"b"="black",
"c"="red",
"d"="pink",
"e"="yellow",
"f"="red",
"g"="green",
"h"="brown",
"i"="purple"
)
ggplot(data, aes(x = Corporations, y = Q1.19, fill = Instruments)) +
geom_bar(position = "fill", stat = "identity") +
coord_polar() +
ggtitle(expression(paste(underline("Quartal 1, 2019")))) +
theme(panel.background = element_rect(colour = "grey"),
panel.border = element_blank(),
panel.grid.major = element_blank(),
panel.grid.minor = element_blank(),
legend.background = element_rect(colour = "grey"),
axis.title.x = element_blank(),
axis.title.y = element_blank(),
axis.text.y=element_blank(),
axis.ticks.x = element_blank(),
axis.ticks.y = element_blank(),
plot.title = element_text(hjust = 0.5, size = 20)) +
scale_x_discrete(labels = wrap_format(10)) +
geom_text(aes(y=sum(Q1.19), label = paste(format(sum(Q1.19), big.mark = ".", scientific = FALSE), "Mio. €"),),
size = 4, position = position_fill(vjust = 0.7)) +
scale_fill_manual(values = list_of_colours_per_instrument)