Change color scheme for aes mapping (ggplot2)

I have the following table:

fst = fread("beng_wbm_92x_win2000.window")
colnames(fst) = c("Chr", "Pos", "Fst")

> fst
       chr     Pos      fst
    1:   0    2500 0.531040
    2:   0    3500 0.818037
    3:   0    5500 0.829916
    4:   0    6500 0.505682
    5:   0    7500 0.639140
   ---
43565:  99 3006500 0.759512
43566: 993   14500 0.693944
43567: 993   15500 0.523174
43568: 993   16500 0.586963
43569: 993   17500 0.858804

I am trying to do a heatmap using:

mine.heatmap <- ggplot(fst, mapping = aes(x = Pos, y = Chr, fill = Fst)) +
  geom_tile() +
  xlab(label = "Pos") +
  ylab(label = "Scaffold") +
  coord_cartesian(ylim=c(-0, 1140))

ggsave(filename = "FST_TajimaD.png", mine.heatmap , width = 5, height = 4, dpi = 300, units = "in", device='png')

But it will plot the data using a scale in blue, where low Fst values are lighter and high darker.

How can I change this color scheme? Maybe invert so that high values are darker, or use a blue to red scheme.enter image description here