x <- 1:100
y <- 1:100
dat <- expand.grid(x, y)
dat$Var3 <- rnorm(10000,0,1)

first <- ggplot(dat, aes(x = Var1, y = Var2)) +
  geom_tile(aes(fill = Var3)) +
  scale_fill_gradient(low = "white", high = "#88398A")

second <- ggplot(dat, aes(x = Var1, y = Var2)) +
  geom_tile(aes(fill = Var3), col = "white") +
  scale_fill_gradient(low = "white", high = "#88398A")

third <- ggplot(dat, aes(x = Var1, y = Var2)) +
  geom_tile(aes(fill = Var3)) +
  scale_fill_gradient(low = "black", high = "#88398A")

ggsave(filename = "light-1", plot = first, dpi = 1200, device = "png")
ggsave(filename = "light-2", plot = second, dpi = 1200, device = "png")
ggsave(filename = "dark", plot = third, dpi = 1200, device = "png")