Customizing colors in geom_arc_bar (ggforce)
For a discrete scale your Code 1 example was nearly there.  Just use scale_fill_manual instead of scale_color_manual.  For more control over how the discrete values map to color values you can name the vector, too.

my_colors &lt;- c("red","white","blue")
ggplot(df) + req_data +
    theme_no_axes() + scale_fill_manual(values=my_colors)
# or, if you want consistent output independent of the order of
# df$discrete_fill:
my_colors &lt;- c("1" = "red", "2" = "white", "dummy" = "blue")
ggplot(df) + req_data +
    theme_no_axes() + scale_fill_manual(values=my_colors)


You can also just use a single call to geom_arc_bar for the whole data frame (rather than a list of call results from each row) and you'll get the same effect.

ggplot(df) +
  geom_arc_bar(aes(x0 = x0, y0 = y0, r0 = 0, r = 0.45, 
               amount = amount, fill = discrete_fill), stat = "pie") +
    theme_no_axes() + scale_fill_manual(values=my_colors)